Search query `repohascommitafter:"january 1 2018"` fails if you have an empty repository

Created by: eugeneiiim

• Sourcegraph version: 3.6.0
• Platform information: N/A

Steps to reproduce:

1. Create an empty repository called my-org/my-empty-repo and allow Sourcegraph to import it.
2. Search for repohascommitafter:"january 1 2018"

Expected behavior:

The query should return results for repos with commits since January 1 2018.

Actual behavior:

Query returns an error: Revision not found: git.blendlabs.com/my-org/my-empty-repo@HEAD (and 2 more)