Alias() drops missing identities with warning

Patch changes

• 6397d78 alias() now validates both identities before enqueuing. Previously alias(None, "user-123") (or an empty-string previous_id) sent a $create_alias event with a null/empty distinct_id, which cannot link anything and just adds an unusable event to the project. Missing identities are now dropped with a warning instead, matching the sdk-specs alias contract. The drop that already happened when no alias target could be resolved now logs a warning too, and a non-string previous_id such as 0 is stringified consistently in both distinct_id and properties.distinct_id. — Thanks @posthog[bot]!