[NDB_BVL_Instrument] Add null value to examiner array

[CamilleBeau]

Brief summary of changes

This PR adds an empty value to the array of examiner names. This was mistakenly taken out in #7462, and is causing the first value to be selected by default in instruments.

Testing instructions (if applicable)

1. Check that there is an empty option in the examiner drop down of instruments, and make sure that no value is selected by default.

[ridz1208]

@CamilleBeau any reason or effect that the removed code was array(''=>'') and you are adding [null=>''] ?

[CamilleBeau]

@CamilleBeau any reason or effect that the removed code was array(''=>'') and you are adding [null=>''] ?

There is no reason. I just put null => '' because that's what we usually do in instruments on CCNA.