Tweak role member's "NULL" searches

Previously, "IS NULL" meant that role doesn't have any members. Sometimes
users actually mean that the role does have members, it's just the specified
field of the members is NULL.

    Requestor.CustomField.Manager IS NULL

This commit tweaks NULL searches and splits it into 2 cases:

* The ticket doesn't have any requestors

    Requestor IS NULL
    Requestor.id IS NULL
    Requestor.Name IS NULL

* At least one of the ticket requestors has NULL in specified user fields.

    Requestor.City IS NULL
    Requestor.CustomField.Manager IS NULL

lib/RT/SearchBuilder/Role/Roles.pm

@@ -306,7 +306,7 @@ sub RoleLimit {
     }
 
     $self->_OpenParen( $args{SUBCLAUSE} ) if $args{SUBCLAUSE};
-    if ( $args{OPERATOR} =~ /^IS(?: NOT)?$/i ) {
+    if ( $args{OPERATOR} =~ /^IS(?: NOT)?$/i && ( $args{FIELD} =~ /^(?:id|Name)$/ ) ) {
         # is [not] empty case
 
         $group_members ||= $self->_GroupMembersJoin( GroupsAlias => $groups );
@@ -571,6 +571,7 @@ sub RoleLimit {
                 if ( $cf->id && $cf->CurrentUserHasRight('SeeCustomField') ) {
                     my $ocfvs = $self->NewAlias('ObjectCustomFieldValues');
                     $self->Join(
+                        TYPE   => 'LEFT',
                         ALIAS1 => $users,
                         FIELD1 => 'id',
                         ALIAS2 => $ocfvs,
@@ -618,6 +619,16 @@ sub RoleLimit {
                     CASESENSITIVE   => 0,
                 );
             }
+
+            $self->Limit(
+                %args,
+                ALIAS           => $users,
+                FIELD           => 'id',
+                OPERATOR        => 'IS NOT',
+                VALUE           => 'NULL',
+                CASESENSITIVE   => 0,
+                ENTRYAGGREGATOR => 'AND',
+            );
         }
     }
     $self->_CloseParen( $args{SUBCLAUSE} ) if $args{SUBCLAUSE};