IsomorphismPartialPermSemigroup/Monoid does not work for a perm group with 0 generators

[wilfwilson]

Observed behaviour

Given a group with 0 generators, such as the symmetric group of degree 1:

gap> IsomorphismPartialPermSemigroup(SymmetricGroup(1));
Error, Usage: cannot create an inverse semigroup with no generators, at /Users/Wilf/GAP/lib/invsgp.gi:167 called from
InverseSemigroup( List( GeneratorsOfGroup( G ), function ( p )
        return AsPartialPerm( p, dom );
    end ) ) at /Users/Wilf/GAP/lib/semipperm.gi:377 called from
<function "unknown">( <arguments> )
gap> IsomorphismPartialPermMonoid(SymmetricGroup(1));
Error, Usage: cannot create an inverse monoid with no generators, at /Users/Wilf/GAP/lib/invsgp.gi:118 called from
InverseMonoid( List( GeneratorsOfGroup( G ), function ( p )
        return AsPartialPerm( p, dom );
    end ) ) at /Users/Wilf/GAP/lib/semipperm.gi:359 called from
<function "unknown">( <arguments> )

Expected behaviour

This should not trigger an error, and should work like this:

gap> IsomorphismPartialPermSemigroup(Group(()));
MappingByFunction( Group(()), <trivial partial perm group of rank 0 with
  1 generator>, function( p ) ... end, <Attribute "AsPermutation"> )
gap> IsomorphismPartialPermMonoid(Group(()));
MappingByFunction( Group(()), <trivial partial perm group of rank 0 with
  1 generator>, function( p ) ... end, <Attribute "AsPermutation"> )

You can't create an inverse semigroup from an empty list, so I'll fix this by having a special case.