Flatc parser support for nullable scalars (#6026)

* Parser support for nullable scalars

* Use older C++ features

* use default element

* Add a test for json, flexbuffers, and null

* test comments and names

Co-authored-by: Casper Neo <cneo@google.com>

include/flatbuffers/idl.h

@@ -296,6 +296,7 @@ struct FieldDef : public Definition {
         shared(false),
         native_inline(false),
         flexbuffer(false),
+        nullable(false),
         nested_flatbuffer(NULL),
         padding(0) {}
 
@@ -314,6 +315,8 @@ struct FieldDef : public Definition {
   bool native_inline;  // Field will be defined inline (instead of as a pointer)
                        // for native tables if field is a struct.
   bool flexbuffer;     // This field contains FlexBuffer data.
+  bool nullable;       // If True, this field is Null (as opposed to default
+                       // valued).
   StructDef *nested_flatbuffer;  // This field contains nested FlatBuffer data.
   size_t padding;                // Bytes to always pad after this field.
 };
@@ -927,6 +930,7 @@ class Parser : public ParserState {
 
   bool SupportsAdvancedUnionFeatures() const;
   bool SupportsAdvancedArrayFeatures() const;
+  bool SupportsNullableScalars() const;
   Namespace *UniqueNamespace(Namespace *ns);
 
   FLATBUFFERS_CHECKED_ERROR RecurseError();