Show a warning for subtitles spread over multiple DASH periods.

yt_dlp/extractor/common.py

@@ -2587,6 +2587,12 @@ class InfoExtractor:
                 elif 'fragments' in f:
                     formats[format_key].setdefault('fragments', []).extend(f['fragments'])
 
+            if len(period['subtitles']) > 0 and len(subtitles) > 0:
+                self.report_warning(bug_reports_message(
+                    'Found subtitles in multiple periods in the DASH manifest; '
+                    'if part of the subtitles are missing,'
+                ), only_once=True)
+
             for sub_lang, sub_info in period['subtitles'].items():
                 subtitles.setdefault(sub_lang, []).extend(sub_info)
 
@@ -2930,8 +2936,7 @@ class InfoExtractor:
                         period_entry['formats'].append(f)
                     elif content_type == 'text':
                         period_entry['subtitles'][lang or 'und'].append(f)
-            if period_entry['formats'] or period_entry['subtitles']:
-                yield period_entry
+            yield period_entry
 
     def _extract_ism_formats(self, *args, **kwargs):
         fmts, subs = self._extract_ism_formats_and_subtitles(*args, **kwargs)