Be prepared to handle empty class loader contexts

When executing run-tests we can hit an empty class loader contexts if the
compilation sources are passed with "-cp" and later removed from the
context.

Be prepared for such case and assume a PathClassLoader context.

Bug: 63977059
Test: ./art/test/run-test --host --optimizing --dev
      test-art-host

(cherry picked from commit 1a509c8e)

Change-Id: I2db637a1fc83beb760e1724edd6138b0614aa19b