prevent a panic in search

Created by: ijt

Fixes #4759 (closed). This is a quick fix to buy us time so we can investigate whether there is a deeper issue.

The key change here is the deletion of two lines in cmd/frontend/graphqlbackend/textsearch.go, so the slices indexed and unindexed are initialized as nil instead of having a precomputed capacity. In the case of unindexed, this eliminates the possibility of getting a negative capacity that could cause a panic. It's unlikely that this will make any real difference in performance, but if there are worries I can add a benchmark, preferably in a follow-up PR.

Test plan: Reproduced the issue with a unit test.