Api fails to check on custom field is null in some cases

You can reproduce this on the dmaster demo site:

• Create a custom field set for events, and add a custom field.
• Go to  the api explorer
• Check for events of type 'fundraiser' with IS NULL for the newly created field
• You won't find any results; existing events are not shown.

You might think that an empty string or something was assigned as the cusotm field of the existing events, but this is not the case, as can be checked by searching on 'IS NOT NULL' on the new custom field.

I think I have a patch for this; I will probably create a PR tonight.