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/common/imagtiff.cpp

@@ -895,7 +895,7 @@ bool wxTIFFHandler::DoCanRead( wxInputStream& stream )
 
     wxString copyright;
     const wxString desc = ver.BeforeFirst('\n', &copyright);
-    copyright.Replace("\n", "");
+    copyright.Replace("\n", wxString());
 
     return wxVersionInfo("libtiff", major, minor, micro, desc, copyright);
 }