API 2.0: After an update an Epic through the UI, updates through the API will fail if a Theme is not provided

Issue Summary

If you create an Epic through the UI, and try to update it via API 2.0, it will work without problems, but if any update is made on it through the UI, the API will fail if the themeId is not provided (even if the Theme is not a required field for the Epic).

Steps to Reproduce

1. Gurantee that the Theme is not required for Epics
2. Create an Epic through the UI
3. Do a PATCH to update it through the API, passing only one field, like the description
4. Now on the UI do an update to the Epic
5. Try to do a PATCH to update the same Epic without passing the themeId

Expected Results

The second update would work as did the first one.

Actual Results

The error "Cannot update epic with id xxxx. themeId is required." is thrown.

Workaround

When doing a PATCH, the themId needs to be provided, like in the example:

[
    {
     "op": "replace" ,
     "path": "/description" ,
     "value":  "your description here"
    },
    {
     "op": "replace" ,
     "path": "/themeId" ,
     "value":  "0"
    },
]