Feature request: option to pass nothing to JobSpawn() onExit callback

[chykcha3]

Description of the problem or steps to reproduce

Function shell.JobSpawn() provide support for omitting onStdout and onStderr arguments (using nil value), but does not provide the same for onExit.
It seems usable to call JobSpawn() without callback on exit, if you do not care when and how your job ends. Without this option it is necessary to create dummy function that does nothing.

[niten94]

A check is added in 831e31d so I think this issue can be closed. There is currently no release that includes the change, but the nightly build may be used.

[dmaluka]

Right, this is fixed by #3554 so the issue can be closed.