fix DB API user_creds_get for non-existent ID

If you pass a non-existent ID to the user_creds_get call, it throws
an exception "TypeError: 'NoneType' object is not iterable", but it
should probably return None to be consistent with other get methods

Partial-Bug: #1266523
Change-Id: I8887afa04d80b91ff4479c6ede3818da4d67d726
Add user_creds_delete to the DB API

Partial-Bug: #1266523
Change-Id: I8c49c6630ecaa856078146e032ba7a2fa61786d4
Merge "fix DB API user_creds_get for non-existent ID"
Merge "Add user_creds_delete to the DB API"