Ensures that primary_key method will return nil when multi-pk

When table has a composite primary key, the `primary_key` method for
sqlite3 and postgresql was only returning the first field of the key.

Ensures that it will return nil instead, as AR dont support composite pks.
Merge pull request #18259 from arthurnn/return_nil_on_multi_pk

Ensures that primary_key method will return nil when multi-pk