gla11y: do not fail when not passed any .ui file

This avoids build failures when a directory uses UIConfig but does not add
any ui file.

Change-Id: I63413f87fc5515ffb9e3dd9baf25c6a4ba70c2be
Reviewed-on: https://gerrit.libreoffice.org/50071Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

bin/gla11y

@@ -186,9 +186,6 @@ def main():
             elif a == "none":
                 Wnone = True
 
-    if not args:
-        sys.exit("%s: no input files" % progname)
-
     for filename in args:
         try:
             tree = ET.parse(filename)