[NDB_BVL_Instrument] Add null value to examiner array (#7847)

This 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.

php/libraries/NDB_BVL_Instrument.class.inc

@@ -1085,7 +1085,9 @@ abstract class NDB_BVL_Instrument extends NDB_Page
                 'examinerID'
             );
         }
-        $examiners = [];
+        $examiners = [
+            null => ''
+        ];
         if (is_array($results) && !empty($results)) {
             foreach ($results AS $eid => $row) {
                 $name = $row['full_name'];