cmLocalXCodeGenerator: return a default string

Instead of using the `strlen`-based constructor.
diff --git a/Source/cmLocalXCodeGenerator.cxx b/Source/cmLocalXCodeGenerator.cxx
index eb05424..9646e66 100644
--- a/Source/cmLocalXCodeGenerator.cxx
+++ b/Source/cmLocalXCodeGenerator.cxx
@@ -31,7 +31,7 @@
   cmGeneratorTarget const*) const
 {
   // No per-target directory for this generator (yet).
-  return "";
+  return std::string{};
 }
 
 void cmLocalXCodeGenerator::AppendFlagEscape(std::string& flags,