Fix dateparser search_dates: also returns None

dateparser.search.search_dates will return None if no parseable date
could be found in the `text` argument.

This is explained in the original docstring as well.

Closes #15538.

Author: MajorTanya

stubs/dateparser/dateparser/search/__init__.pyi

@@ -13,12 +13,12 @@ def search_dates(
     settings: Settings | dict[str, Any] | None,
     add_detected_language: Literal[True],
     detect_languages_function: _DetectLanguagesFunction | None = None,
-) -> list[tuple[str, datetime, str]]: ...
+) -> list[tuple[str, datetime, str]] | None: ...
 @overload
 def search_dates(
     text: str,
     languages: list[str] | tuple[str, ...] | AbstractSet[str] | None = None,
     settings: Settings | dict[str, Any] | None = None,
     add_detected_language: Literal[False] = False,
     detect_languages_function: _DetectLanguagesFunction | None = None,
-) -> list[tuple[str, datetime]]: ...
+) -> list[tuple[str, datetime]] | None: ...