DEV-27658 Replace null custom field value with empty string (#93)

* docs: add description when custom field has empty values

Co-authored-by: Patrick Nyakoria <patrick.nyakoria@acrolinx.com>

apiary.apib

@@ -1334,6 +1334,9 @@ To display the list in ascending or descending order respectively, you can add a
 - The query parameter `licenseType` sorts by two fields: `licenseType` and `licenseStatus`.
 - To sort by a custom field, the sort value needs to include `customFields`, `.`, and the `{key}`. The key is the name of the custom field.
 
+When you sort by `customFields` in ascending order, empty `customFields` values will appear at the beginning of the list. If you sort
+in descending order, empty values will appear at the end of the list.
+
 To make sure that the sorting behavior for custom fields is consistent, the following special characters can be sorted: `` !@$^*()_+-=,./;'[]<>?:|~` ``
 If you use any additional special characters, the sorting behavior may be inconsistent.