Only seed initial Debian package database if key file is available.

If the Debian package detected no keyring database on installation it
would seed the database with my key, from
/usr/share/doc/onak/noodles.key.gz. This is against Debian policy 12.3 -
packages cannot require files from /usr/share/doc/ to function. Only
seed the database if the file exists, avoiding issues installing when
skipping /usr/share/doc/

Fixes Debian bug #710665.