auth: Fix getTSIGKeys() return value in GSQL and LMDB backends

auth: Fix getTSIGKeys() return value in GSQL and LMDB backends

These Bind and remote backends return true when at least a TSIG key
is returned, as expected by the API and pdnsutil code, but the GSQL
backend returned false if at least a key was returned and true
otherwise. The LMDB backend always returned false.
This caused `pdnsutil b2b-migrate` not to migrate TSIG keys from a
GSQL or LMDB backend, amongst other things.