Fix Filters.regex failing in case the value is None

pyrogram/client/filters/filters.py

@@ -323,6 +323,7 @@ class Filters:
             else:
                 raise ValueError("Regex filter doesn't work with {}".format(type(update)))
 
+            if value:
                 update.matches = list(flt.p.finditer(value)) or None
 
             return bool(update.matches)