filter: Restore default priority of 0 for missing sequences
filter: Restore default priority of 0 for missing sequences

Fixes a regression introduced by "Allow exception to bubble up when
priority file is bad." (63d6cb2) which accidentally changed the return
type of read_priority_scores() from defaultdict(float) to dict.

The calling code looks up sequences unconditionally in the priorities
dictionary and thus defaultdict(float) provides an appropriate fallback
value when no priority is explicitly given in the provided file.

nextstrain/augur,nextstrain/augur,blab/nextstrain-augur,nextstrain/augur