Allow clearkey tests to run if no vendor modules

DRM cannot skip all tests when vendor modules are
not present. Currently even the clearkey tests are
skipping when there is no vendor lib. This can be
replicated by just pushing the DRM VTS test binary
build from master (or DR or MR) and executing the
binary without VTS. All test cases skip (including
clearkey).

This change prevents exiting when no vendor
modules are found.

Test: Push the test binary and confirm that the
clearkey tests run.

bug:64459011
Change-Id: Ic5f554d9b223d6ca4bf26f92f072254cc18f4620