[prosiebensat1] Make upload_date optional

The site has a crazy incorrect date in German formatting, but the correct date is nowhere to be found.

youtube_dl/extractor/prosiebensat1.py

@@ -145,7 +145,6 @@ class ProSiebenSat1IE(InfoExtractor):
                 'ext': 'mp4',
                 'title': 'Kurztrips zum Valentinstag',
                 'description': 'md5:8ba6301e70351ae0bedf8da00f7ba528',
-                'upload_date': '20130206',
                 'duration': 307.24,
             },
             'params': {
@@ -240,7 +239,7 @@ class ProSiebenSat1IE(InfoExtractor):
         thumbnail = self._og_search_thumbnail(page)
 
         upload_date = unified_strdate(self._html_search_regex(
-            self._UPLOAD_DATE_REGEXES, page, 'upload date', fatal=False))
+            self._UPLOAD_DATE_REGEXES, page, 'upload date', default=None))
 
         formats = []