Correctly handle empty vector when printing

before this CL, an empty vector passed to 'dumpVector' would cause a
crash (in the good case) and undefined behaviour otherwise.

In this CL, fix this function.

Bug: 211379801
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I93f4defb947f41d68f0ec22bd35b6c069fbc15e4