Fix FactorCosetAction(G, G) for perm. groups

[ThomasBreuer]

Catch the case of an action with trivial image in DoFactorCosetActionPerm, and delegate to DoFactorCosetAction in this case.

resolves #5902

(Just for curiosity:
The code for DoFactorCosetAction and DoFactorCosetActionPerm deals with the case that the second argument is a list (not a group), but only in order to call Error if this list is empty. The functions called later, such as Core and ActionRefinedSeries, do not accept a list as the value of this argument.
Is this list situation meaningful at all?)

[ThomasBreuer]

I do not understand the remark that the added lines are not covered by tests.
The added test in teststandard should execute these lines of code, I have checked this locally.
What am I missing?

[lgoettgens]

I do not understand the remark that the added lines are not covered by tests. The added test in teststandard should execute these lines of code, I have checked this locally. What am I missing?

If we have a look at the codecov webpage (https://app.codecov.io/gh/gap-system/gap/pull/5903/blob/lib/csetperm.gi#L886), it there states that your added lines are indeed covered (aka are green), while the error case two lines above is uncovered. My assumption is that there is a small offset somewhere in the codecov-to-github interface that mistakenly reports this