Improve viewing of empty semigroups

[wilfwilson]

Currently, unexpected errors can occur when viewing empty semigroups:

gap> S := FullTransformationMonoid(2);;
gap> Subsemigroup(S, []);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `DegreeOfTransformationCollection' on 1 \
arguments at /Users/Wilf/gap/lib/methsel2.g:250 called from
DegreeOfTransformationCollection( GeneratorsOfSemigroup( s )
 ) at /Users/Wilf/gap/lib/semitran.gi:97 called from
DegreeOfTransformationSemigroup( S ) at /Users/Wilf/gap/lib/semitran.gi:28 called from
SemigroupViewStringSuffix( S ) at /Users/Wilf/gap/lib/semigrp.gi:97 called from
ViewString( obj ) at /Users/Wilf/gap/lib/object.gi:420 called from
SHELL( GetBottomLVars(  ), false, false, 3, true, prompt, function (  )
      if IsBound( OnGAPPromptHook ) and IsFunction( OnGAPPromptHook ) then
          OnGAPPromptHook(  );
      else
          return;
      fi;
      return;
  end, "*stdin*", "*stdout*", true ); at /Users/Wilf/gap/lib/session.g:61 called from
...  at *stdin*:2
type 'quit;' to quit to outer loop
brk>

Empty semigroups are well-supported by neither the GAP library nor the Semigroups package. Making everything work with empty semigroups is not my immediate goal. However, since it's possible to create empty semigroups in GAP, I think it makes sense to increase their support, at least where it is easy to do so, like in this PR.

After this PR:

gap> S := FullTransformationMonoid(2);;
gap> Subsemigroup(S, []);
<empty transformation semigroup>

[codecov]

Codecov Report

Merging #3052 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3052      +/-   ##
==========================================
+ Coverage   82.75%   82.99%   +0.24%     
==========================================
  Files         682      684       +2     
  Lines      337565   343262    +5697     
==========================================
+ Hits       279346   284904    +5558     
- Misses      58219    58358     +139

Impacted Files	Coverage Δ
lib/semigrp.gi	75.07% <100%> (+0.15%)	⬆️
lib/magma.gi	77.98% <100%> (+0.11%)	⬆️
src/vector.c	94.46% <0%> (-5.54%)	⬇️
src/hookintrprtr.c	67.34% <0%> (-4.88%)	⬇️
src/iostream.c	62.35% <0%> (-3.5%)	⬇️
src/lists.c	73.39% <0%> (-3.48%)	⬇️
src/vecffe.c	72.02% <0%> (-2.5%)	⬇️
src/records.c	80.9% <0%> (-2.34%)	⬇️
src/c_type1.c	83.96% <0%> (-2.29%)	⬇️
src/sysfiles.c	40.59% <0%> (-1.99%)	⬇️
... and 81 more

[fingolfin]

LGTM