Do not use !str for checking wxString emptiness

While wxString::operator!() is a documented alternative to .empty() since
25 years, using it in just a handful of places in the codebase is not
consistent.

!str is arguably not very readable either, because std::string does not
have such an operator, and neither does e.g. Qt's QString. People would
generally except ! in the context of any kind of strings to check for null
pointer (char*), or an optional with a missing value (std::optional).

samples/widgets/textctrl.cpp

@@ -930,7 +930,7 @@ void TextWidgetsPage::OnButtonLoad(wxCommandEvent& WXUNUSED(event))
     pathlist.Add("../../../samples/widgets");
 
     wxString filename = pathlist.FindValidPath("textctrl.cpp");
-    if ( !filename )
+    if ( filename.empty() )
     {
         wxLogError("File textctrl.cpp not found.");
     }