Use wxEmptyString instead of wxString().

To keep uniform notation of empty strings.

src/propgrid/editors.cpp

@@ -1042,7 +1042,7 @@ wxWindow* wxPGChoiceEditor::CreateControlsBase( wxPropertyGrid* propGrid,
 #endif
     cb->Create(ctrlParent,
                wxID_ANY,
-               wxString(),
+               wxEmptyString,
                po,
                si,
                labels,

src/propgrid/property.cpp

@@ -1007,7 +1007,7 @@ wxString wxPGProperty::ValueToString( wxVariant& WXUNUSED(value),
                                       int argFlags ) const
 {
     wxCHECK_MSG( GetChildCount() > 0,
-                 wxString(),
+                 wxEmptyString,
                  "If user property does not have any children, it must "
                  "override GetValueAsString" );