Fix FileFinder for empty result lists without checksum generation

Summary:
If you try to find something that doesn't exist, it currently gives you back the
empty string as the only result. This is wrong; the checksum case gets it right.
Share logic.

Test Plan:
Tried to FileFinder a nonexistent path without checksumming.

Reviewed By: aran
Reviewers: tuomaspelkonen, aran, jungejason
CC: aran
Differential Revision: 268