Fix WHERE clause for keyword search queries with empty keywords

When the keyword parameter is empty, the AND clause has to be omitted,
otherwise we get an SQL syntax error:

... WHERE PackageBases.PackagerUID IS NOT NULL AND () ...

This got broken in commit 9e30013a
Author: Kevin Morris <kevr.gtalk@gmail.com>
Date:   Sun Jul 5 18:19:06 2020 -0700

Support conjunctive keyword search in RPC interface

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>
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