fix(bttest): Reject empty regex row filters

[adamroyjones]

This fixes GitHub issue #6519; see that issue for further commentary.

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[adamroyjones]

I've now rebased the branch onto the current version of main.

I don't know how the CI system is set up in this repo. Is the "kokoro" step causing it to block?

[telpirion]

I've now rebased the branch onto the current version of main.

I don't know how the CI system is set up in this repo. Is the "kokoro" step causing it to block?

Yeah, the CI can cause it to block. I'll try to get this PR unstuck.