Fix AtestToolsUnittests#test_index_targets failure

The new version locate command exit code will always be 0 even not
result found. Change the logic to check the command out as empty string
instead.

Bug: 230436363
Test: atest atest_unittests --host --no-bazel-mode
Change-Id: I3ac12fda3cea38360c63797b901a111ed977ce8c
Merged-In: I0b52d493d6de0eaadf9777fb95f4d5e432930d4b
1 file changed