RankOfPartialPermSemigroup fails for a partial perm group with no generators

[wilfwilson]

Using GAP master branch:

gap> G := Group([], PartialPerm([1, 2]));;
gap> RankOfPartialPermSemigroup(G);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `RankOfPartialPermCollection' on 1 argum\
ents at /Users/Wilf/gap/lib/methsel2.g:250 called from
RankOfPartialPermCollection( GeneratorsOfGroup( S )
 ) at /Users/Wilf/gap/lib/semipperm.gi:104 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:5
type 'quit;' to quit to outer loop
brk>

This should return 2. I noticed this when trying to view the group:

gap> G;
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `RankOfPartialPermCollection' on 1 argum\
ents at /Users/Wilf/gap/lib/methsel2.g:250 called from
RankOfPartialPermCollection( GeneratorsOfGroup( S )
 ) at /Users/Wilf/gap/lib/semipperm.gi:104 called from
RankOfPartialPermSemigroup( S ) at /Users/Wilf/gap/lib/semipperm.gi:30 called from
SemigroupViewStringSuffix( S ) at /Users/Wilf/gap/lib/semigrp.gi:166 called from
ViewString( G ) at /Users/Wilf/gap/lib/grp.gi:4321 called from
SetUserHasQuit( 1 ); at /Users/Wilf/gap/lib/error.g:328 called from
...  at *stdin*:5
type 'quit;' to quit to outer loop
brk>

For a partial perm group G, RankOfPartialPermSemigroup should probably return RankOfPartialPerm(One(G));

[fingolfin]

This also affects older GAP versions, like 4.8.