[NDB_BVL_Instrument] Only update instrument table if $values not empty

[zaliqarosli]

Brief summary of changes

This PR fixes the _save() method to call db update only if $values is not empty. This case pops up when you have a subtest with only static score elements. No error should be thrown so that $this->score() can eventually be called within the save() method.

Testing instructions (if applicable)

1. Mock an instrument subtest with only static score fields or test with one that you already have
2. on aces/main, click 'Save Data' on the front-end on that subtest. You will get a SQL error page.
3. on this PR branch, repeat the above, and no errors. you can add a .score file to test that the scoring still works even if no values are saved