[BUG] Does not handle undefined return value from signal-exit's onExit() function

[kayahr]

According to the documentation of signal-exit the onExit function always returns a function. But according to the actual source code this is not true. The function can return undefined in specific circumstances (I encounter this within Electron) and write-file-atomic fails when trying to call this undefined function.

I guess the function call should be wrapped with a null check.