User is able to merge a single actor

User is able to merge a single actor using the Merge Actor tool.

This is a regression as you could not merge a single actor in 4.12. Not sure if this was an intended change or not.

Occurring in 4.13
Not occurring in 4.12

1. Open any project in the editor
2. Drag a static mesh into the level
3. Open the Merge Actors window by going to Window->Developer Tools->Merge Actors
4. Select only the static mesh you dragged into the level

Result: User is able to Merge Actor with only a single actor selected

Expected: Only able to merge actors with two or more actors selected