On startup, OpenDKIM verifies if the SigningTable doesn't reference any missing
KeyTable entries. However, if the KeyTable is put in an SQL database, and the
key is missing for a given domain, this gives the following error on startup,
and refuses to startup:

opendkim: /etc/opendkim.conf: could not find valid key record "" in KeyTable

This error is quite incomprehensible, and certainly doesn't point to the fact
that startup fails due to invalid data in the database. Please change the error
message to something like this:

opendkim: failed to find valid key record "" in KeyTable for domain "example.com"
opendkim: refusing to start due to invalid data in database

Thanks in advance!