[fuchsia_modular] Fix assertion: when 'wantsRichSuggestion' is true, the
headline can be an empty string.

This had previously been fixed before the SDK but it looks like the
change didn't make it over: http://go/fxr/223492

Tested: fx ota, see krohnkite stops throwing exceptions.
Change-Id: I3082bea6d75f0c9a83b56a558493d7f547d57dc1
diff --git a/public/dart/fuchsia_modular/lib/src/proposal/proposal.dart b/public/dart/fuchsia_modular/lib/src/proposal/proposal.dart
index c3eed8c..d1dec7a 100644
--- a/public/dart/fuchsia_modular/lib/src/proposal/proposal.dart
+++ b/public/dart/fuchsia_modular/lib/src/proposal/proposal.dart
@@ -33,7 +33,8 @@
     int color = 0x000000,
     void Function(String, String) onProposalAccepted,
   })  : assert(id != null && id.isNotEmpty),
-        assert(headline != null && headline.isNotEmpty),
+        assert(headline != null),
+        assert(headline.isNotEmpty || wantsRichSuggestion),
         super(
           id: id,
           storyName: storyName,