Support missing instrumented_for dependency when ALLOW_MISSING_DEPENDENCIES=true

Don't panic when the instrumented_for dependency is missing or when
instrumentedApp or files generated from instrumentedApp are nil.  Fixes
unbundled tradefed build.

Fixes: 283451533
Test: tapas google-tradefed-all && BUILD_BROKEN_DISABLE_BAZEL=true nothing on tradefed branch
Change-Id: Ibcd0da75fc55c7c0a697e98ae6bdd9fd3662259c