[CiviReport] Add "is null" and "is not null" to OP_MULTISELECT_SEPARATOR

Overview

In a report that uses a OP_MULTISELECT_SEPARATOR widget as a filter it is impossible to filter by "Is empty (Null)" / "Is not empty (Null)".

Current behaviour

In a report that uses a OP_MULTISELECT_SEPARATOR widget as a filter it is impossible to filter by "Is empty (Null)" / "Is not empty (Null)".

Proposed behaviour

Allow for "Is empty (Null)" / "Is not empty (Null)" options for OP_MULTISELECT_SEPARATOR filters.

Comments

This is a simple, straightforward change: In Form.php, function getOperationPair, lines 1796 - 1804 add

      case CRM_Report_Form::OP_MULTISELECT_SEPARATOR:
        // use this operator for the values, concatenated with separator. For e.g if
        // multiple options for a column is stored as ^A{val1}^A{val2}^A
        $result = [
          'mhas' => ts('Is one of'),
          'mnot' => ts('Is not one of'),
          // Add these two lines
          'nll' => ts('Is empty (Null)'),
          'nnll' => ts('Is not empty (Null)'),
        ];
        return $result;

I have tested this for one report and it works as expected.