Don't append just a dot in wxGTK wxFileDialog if no default extension.

Don't append just a dot in wxGTK wxFileDialog if no default extension.

We always appended the default extension (for the currently selected pattern)
to the file name but this was the wrong thing to do if there was no extension
at all as this resulted in appending just a dot to the filename. So only do it
if there is an extension to append.

Closes #15285.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74307 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775