Check for valid Diagnostic

Change-Id: Icfce44a373d56f008027330f9b1a2d446f4d2adb
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

plugins/clangstaticanalyzer/clangstaticanalyzerdiagnostic.cpp

@@ -46,5 +46,10 @@ bool ExplainingStep::isValid() const
     return location.isValid() && !ranges.isEmpty() && !message.isEmpty();
 }
 
+bool Diagnostic::isValid() const
+{
+    return !description.isEmpty();
+}
+
 } // namespace Internal
 } // namespace ClangStaticAnalyzer