Expand std::string_view test coverage

Fill in cases which weren't being tested for some target languages.
diff --git a/Examples/test-suite/csharp/cpp17_string_view_runme.cs b/Examples/test-suite/csharp/cpp17_string_view_runme.cs
index 7b9ee07..a7b54b7 100644
--- a/Examples/test-suite/csharp/cpp17_string_view_runme.cs
+++ b/Examples/test-suite/csharp/cpp17_string_view_runme.cs
@@ -80,5 +80,18 @@
 
         if (Structure.ConstStaticMemberString != "const static member string")
           throw new Exception("ConstStaticMemberString test");
+
+        if (cpp17_string_view.stdstringview_empty() != "")
+          throw new Exception("stdstringview_empty test");
+        if (cpp17_string_view.c_empty() != "")
+          throw new Exception("c_empty test");
+        if (cpp17_string_view.c_null() != null)
+          throw new Exception("c_null test");
+        if (cpp17_string_view.get_null(cpp17_string_view.c_null()) != null)
+          throw new Exception("get_null c_null test");
+        if (cpp17_string_view.get_null(cpp17_string_view.c_empty()) != "non-null")
+          throw new Exception("get_null c_empty test");
+        if (cpp17_string_view.get_null(cpp17_string_view.stdstringview_empty()) != "non-null")
+          throw new Exception("get_null stdstringview_empty test");
     }
 }
diff --git a/Examples/test-suite/java/cpp17_string_view_runme.java b/Examples/test-suite/java/cpp17_string_view_runme.java
index 903970e..6636a52 100644
--- a/Examples/test-suite/java/cpp17_string_view_runme.java
+++ b/Examples/test-suite/java/cpp17_string_view_runme.java
@@ -88,5 +88,18 @@
 
       if (!Structure.getConstStaticMemberString().equals("const static member string"))
         throw new Exception("ConstStaticMemberString test");
+
+      if (!cpp17_string_view.stdstringview_empty().equals(""))
+        throw new Exception("stdstringview_empty test");
+      if (!cpp17_string_view.c_empty().equals(""))
+        throw new Exception("c_empty test");
+      if (cpp17_string_view.c_null() != null)
+        throw new Exception("c_null test");
+      if (cpp17_string_view.get_null(cpp17_string_view.c_null()) != null)
+        throw new Exception("get_null c_null test");
+      if (!cpp17_string_view.get_null(cpp17_string_view.c_empty()).equals("non-null"))
+        throw new Exception("get_null c_empty test");
+      if (!cpp17_string_view.get_null(cpp17_string_view.stdstringview_empty()).equals("non-null"))
+        throw new Exception("get_null stdstringview_empty test");
   }
 }
diff --git a/Examples/test-suite/lua/cpp17_string_view_runme.lua b/Examples/test-suite/lua/cpp17_string_view_runme.lua
index b30d992..dd6215a 100644
--- a/Examples/test-suite/lua/cpp17_string_view_runme.lua
+++ b/Examples/test-suite/lua/cpp17_string_view_runme.lua
@@ -48,3 +48,10 @@
 assert(Structure.ConstStaticMemberString=="const static member string")
 
 test_const_reference_returning_void("foo")
+
+assert(stdstringview_empty()=="")
+assert(c_empty()=="")
+assert(c_null()==nil)
+assert(get_null(c_null())==nil)
+assert(get_null(c_empty())=="non-null")
+assert(get_null(stdstringview_empty())=="non-null")
diff --git a/Examples/test-suite/php/cpp17_string_view_runme.php b/Examples/test-suite/php/cpp17_string_view_runme.php
index 5f756bb..ff9b4d2 100644
--- a/Examples/test-suite/php/cpp17_string_view_runme.php
+++ b/Examples/test-suite/php/cpp17_string_view_runme.php
@@ -41,4 +41,11 @@
 
 cpp17_string_view::test_const_reference_returning_void("foo");
 
+check::equal(cpp17_string_view::stdstringview_empty(), "", "stdstringview_empty test");
+check::equal(cpp17_string_view::c_empty(), "", "c_empty test");
+check::isnull(cpp17_string_view::c_null(), "c_null test");
+check::isnull(cpp17_string_view::get_null(cpp17_string_view::c_null()), "get_null c_null test");
+check::equal(cpp17_string_view::get_null(cpp17_string_view::c_empty()), "non-null", "get_null c_empty test");
+check::equal(cpp17_string_view::get_null(cpp17_string_view::stdstringview_empty()), "non-null", "get_null stdstringview_empty test");
+
 check::done();
diff --git a/Examples/test-suite/tcl/cpp17_string_view_runme.tcl b/Examples/test-suite/tcl/cpp17_string_view_runme.tcl
index a63c25e..1b6fec8 100644
--- a/Examples/test-suite/tcl/cpp17_string_view_runme.tcl
+++ b/Examples/test-suite/tcl/cpp17_string_view_runme.tcl
@@ -33,3 +33,14 @@
 if {$Structure_ConstStaticMemberString != "const static member string"} { error "bad ConstStaticMemberString"}
 
 test_const_reference_returning_void "foo"
+
+if {[stdstringview_empty] != ""} { error "bad stdstringview_empty test" }
+if {[c_empty] != ""} { error "bad c_empty test" }
+# FIXME: [c_null] seems to give an empty string currently, but Tcl doesn't have
+# a real NULL value and the string "NULL" we used for elsewhere for NULL
+# pointers doesn't work well here as it's indistinguishable from the string
+# "NULL" being returned.
+#if {[c_null] != "NULL"} { error "bad c_null test" }
+#if {[get_null [c_null]] != "NULL"} { error "bad get_null c_null test" }
+if {[get_null [c_empty]] != "non-null"} { error "bad get_null c_empty test" }
+if {[get_null [stdstringview_empty]] != "non-null"} { error "bad get_null stdstringview_empty test" }