github_commit fails when there is no commit or repo is empty. Fixes #181

[LalitLab]

Example query results

Results

> select * from github_commit where repository_full_name = 'rajmohanty/onlyissue'
+----------------------+-----+--------------+-------------+----------+--------------+--------+-----------------+----------------+-------+----------+---------+---------+---------+-------+-----+------+
| repository_full_name | sha | author_login | author_date | verified | comments_url | commit | committer_login | committer_date | files | html_url | message | node_id | parents | stats | url | _ctx |
+----------------------+-----+--------------+-------------+----------+--------------+--------+-----------------+----------------+-------+----------+---------+---------+---------+-------+-----+------+
+----------------------+-----+--------------+-------------+----------+--------------+--------+-----------------+----------------+-------+----------+---------+---------+---------+-------+-----+------+

Time: 1691.5ms.