Fix `fastboot flash-all` on Nexus 7.

The Nexus 7 bootloader just returns the empty string for unknown
variables, which confused the new snapshot code.

Bug: https://issuetracker.google.com/158232468
Test: no Nexus 7 available during covid-19 :-(
Change-Id: I35ff8889b27944e8b7426eca4f513d9fa562c6d4