Return null BSTR from wxIAccessible if string returned from wxAccessible method is empty

wxIAccessible should return a NULL BSTR to the accessibility framework if strings returned from wxIAccessible::get_accName() and get_accValue() are empty because this convention is already applied to the other methods returning string values, like get_accHelp(), get_accDescription().

docs/changes.txt

@@ -143,6 +143,8 @@ wxMSW:
   errors in wxAccessible methods.
 - Return DISP_E_MEMBERNOTFOUND error code from wxIAccessible if wxAccessible
   methods return wxAccStatus::wxACC_NOT_SUPPORTED.
+- Return null BSTR from wxIAccessible if string returned from wxAccesible
+  method is empty.
 
 wxOSX: