scylla-gdb.py: find_db(): don't return current shard's database for shard=0
scylla-gdb.py: find_db(): don't return current shard's database for shard=0

The `shard` parameter of `find_db()` is optional and is defaulted to
`None`. When missing, the current shard's database instance is returned.
The problem is that the if condition checking this uses `not shard`,
which also evaluates to `True` if `shard == 0`, resulting in returning
the current shard's database instance for shard 0. Change the condition
to `shard is None` to avoid this.

Fixes: #7016
Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200812091546.1704016-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla