Accidentally, I've tried to push PKGBUILD without `url` field being present. There was following reply from the server:

Counting objects: 4, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 814 bytes | 0 bytes/s, done.
Total 4 (delta 0), reused 0 (delta 0)
remote: Traceback (most recent call last):
remote: File "hooks/update", line 306, in <module>
remote: save_srcinfo(srcinfo, db, cur, user)
remote: File "hooks/update", line 89, in save_srcinfo
remote: pkginfo['pkgdesc'], pkginfo['url']])
remote: File "/usr/lib/python3.4/site-packages/mysql/connector/cursor.py", line 507, in execute
remote: self._handle_result(self._connection.cmd_query(stmt))
remote: File "/usr/lib/python3.4/site-packages/mysql/connector/connection.py", line 722, in cmd_query
remote: result = self._handle_result(self._send_cmd(ServerCmd.QUERY, query))
remote: File "/usr/lib/python3.4/site-packages/mysql/connector/connection.py", line 640, in _handle_result
remote: raise errors.get_exception(packet)
remote: mysql.connector.errors.IntegrityError: 1048 (23000): Column 'URL' cannot be null
remote: error: hook declined to update refs/heads/master
Toaur@aur4.archlinux.org/go-makepkg.git"> ssh://aur@aur4.archlinux.org/go-makepkg.git
! [remote rejected] master -> master (hook declined)
error: failed to push some refs to 'ssh://aur@aur4.archlinux.org/go-makepkg.git'