Use wxString() instead of "" for empty strings

This will allow this code to work even when implicit conversion from
"const char*" is disabled in wxString and is already marginally more
efficient even now.

See https://github.com/wxWidgets/wxWidgets/pull/782

src/generic/richtooltipg.cpp

@@ -77,7 +77,7 @@ public:
         }
         //else: Simply don't show any icon.
 
-        wxStaticText* const labelTitle = new wxStaticText(this, wxID_ANY, "");
+        wxStaticText* const labelTitle = new wxStaticText(this, wxID_ANY, wxString());
         labelTitle->SetLabelText(title);
 
         wxFont titleFont(titleFont_);