Steps to reproduce:
1. Set up Perforce server and Unity Editor integration
2. In Unity create a random file
3. Through P4 mark the file for exclusive check out
4. Back in the editor, through the VC context menu check out the file
5. In the VC context menu select Unlock
6. It does nothing

Expected: If it's available to select, it should do something, otherwise, it should be marked as unclickable
Actual: You can click it and nothing happens

Reproduced on: 2018.1.0a3, 2017.3.0b8, 2017.2.0p1, 2017.1.2p2