[NDB_BVL_Instrument] Only update instrument table if $values not emp…

…ty (#7124)

This 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.

php/libraries/NDB_BVL_Instrument.class.inc

@@ -794,7 +794,7 @@ abstract class NDB_BVL_Instrument extends NDB_Page
             $values = Utility::nullifyEmpty($values, $key);
         }
 
-        if ($this->jsonData !== true) {
+        if ($this->jsonData !== true && !empty($values)) {
             // If the instrument is saving as JSON into the Data column, the
             // table may not exist, so don't try and update it.
             $db->update(