Blender Git Loki

Fix T76179: Unable to select render passes when a render has fewer
passes than one in another slot

If a particular pass is not available in a slot we are switching to,
still show the menu, but with a blank name for the currently selected
item so that the user can change it to a valid value.

thx @brecht for providing the standard way Blender deals with these
kinds of situations.

Maniphest Tasks: T76179

Differential Revision: https://developer.blender.org/D7552