Clicking off (blur) a nullable AppCombo sets raw markup in field

[vanstyn]

This was observed to happen within an Add Form on a single rel column ('combo' auto_editor_type) when typing in some invalid/not found text, and then clicking off the field. It should be retaining the null/empty value, but it is instead inserting the literal markup as a string:

<span class="ra-combo-select-none">(None)</span>