fix(core): use "is null" instead of "= null" when building SQL

Keep NSNull instances for values in GCSFolder and let [EOQualifier(GCS)
_gcsAppendToString:] properly format the key/value qualifier that
involves a NULL.

This fixes various issues, including missing unique recurrent events in
attendees calendars.

SOPE/GDLContentStore/GCSFolder.m

@@ -457,7 +457,7 @@ static GCSStringFormatter *stringFormatter = nil;
 
   field = [qualifier key];
   attribute = [self _attributeForColumn: field];
-  if (attribute)
+  if (attribute && [[qualifier value] isNotNull])
     {
       formattedValue = [adaptor formatValue: [qualifier value]
                                forAttribute: attribute];