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

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.