fix(mcp): use is not None to cache empty tools list correctly

[rmotgi1227]

When an MCP server returns an empty tools list, self._tools_list is [] which is falsy. The cache check

if self.cache_tools_list and not self._cache_dirty and self._tools_list:

treats an empty cached list the same as an uncached one, causing a redundant re-fetch on every subsequent list_tools() call even when cache_tools_list=True and the cache is clean.

The field is initialized to None (line 600) to mean "not yet fetched," so the correct guard is is not None.

[seratch]

An empty list here is not expected plus if that happens, skipping the saved empty data should be fine