[extractor] Fix empty `BaseURL` in MPD

Closes #4113

yt_dlp/extractor/common.py

@@ -63,6 +63,7 @@ from ..utils import (
     str_to_int,
     strip_or_none,
     traverse_obj,
+    try_call,
     try_get,
     unescapeHTML,
     unified_strdate,
@@ -2820,7 +2821,7 @@ class InfoExtractor:
                     base_url = ''
                     for element in (representation, adaptation_set, period, mpd_doc):
                         base_url_e = element.find(_add_ns('BaseURL'))
-                        if base_url_e is not None:
+                        if try_call(lambda: base_url_e.text) is not None:
                             base_url = base_url_e.text + base_url
                             if re.match(r'^https?://', base_url):
                                 break