#JavaScript ES6, 85 bytes
As an anonymous function
a=>["Blinky","Inky","Pinky","Clyde"].filter(c=>!a.includes(c)||a.includes("Pac-Man"))
Today I learned about this filter function. Fun!
15 bytes saved thanks to Neil.
Usage:
(a=>["Blinky","Inky","Pinky","Clyde"].filter(c=>!a.includes(c)||a.includes("Pac-Man")))(["Pac-Man"])
> ["Blinky","Inky","Pinky","Clyde"]
(a=>["Blinky","Inky","Pinky","Clyde"].filter(c=>!a.includes(c)||a.includes("Pac-Man")))(["Pinky"])
> ["Blinky","Inky","Clyde"]
(a=>["Blinky","Inky","Pinky","Clyde"].filter(c=>!a.includes(c)||a.includes("Pac-Man")))([])
> ["Blinky","Inky","Pinky","Clyde"]
