Use wxString::empty() function calls to determine if string is empty in wxPG.

Use this dedicated function instead of explicitly check if wxString::length() returns non-zero value.

src/propgrid/propgridpagestate.cpp

@@ -1692,7 +1692,7 @@ bool wxPropertyGridPageState::PrepareToAddItem( wxPGProperty* property,
 
     if ( scheduledParent && !scheduledParent->IsCategory() )
     {
-        wxASSERT_MSG( property->GetBaseName().length(),
+        wxASSERT_MSG( !property->GetBaseName().empty(),
                       "Property's children must have unique, non-empty names within their scope" );
     }