gh-103878: Return a consistent empty value from cancelled file dialogs (GH-152435)

gh-103878: Return a consistent empty value from cancelled file dialogs (GH-152435)

On cancellation Tcl may report the empty result as '', () or b'' depending
on the platform and Tk version. Normalize it so that askopenfilename(),
asksaveasfilename() and askdirectory() always return '' and
askopenfilenames() always returns ().

Open._fixresult() now distinguishes single from multiple results by the
'multiple' option rather than the result type.

Co-authored-by: Christopher Chavez <chrischavez@gmx.us>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>