Search: run repo subset symbol search on empty pattern
Created by: camdencheek
Currently, searches like type:symbol repo:^github\.com/sourcegraph/sourcegraph$ count:all do not return any results.
It seems that "skipRepoSubsetSearch" specifically means "skipUnindexedRepoSubsetSearch", but we were using it to determine whether to skip indexed symbol subset search. This modifies the logic to still run zoekt subset search on symbols even when skipRepoSubsetSearch is true.