Use wxNullBitmap explicitly instead of wxPG_NULL_BITMAP.

wxPG_NULL_BITMAP macro is not useful because is used only once. In the rest of wxPG code wxNullBitmap is used explicitly many times so for the sake of consistency we can resign from using this macro.

include/wx/propgrid/propgriddefs.h

@@ -235,7 +235,9 @@ class wxPGValidationInfo;
 
 // This is the value placed in wxPGProperty::sm_wxPG_LABEL
 #define wxPG_LABEL_STRING       wxS("@!")
+#if WXWIN_COMPATIBILITY_3_0
 #define wxPG_NULL_BITMAP        wxNullBitmap
+#endif // WXWIN_COMPATIBILITY_3_0
 #define wxPG_COLOUR_BLACK       (*wxBLACK)
 
 /** Convert Red, Green and Blue to a single 32-bit value.