Ensure TrivialGroup(IsPermGroup) has zero generators

[fingolfin]

This addresses #1239 (or at least parts of it).

[codecov]

Codecov Report

Merging #1247 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1247      +/-   ##
==========================================
+ Coverage   60.61%   60.62%   +<.01%     
==========================================
  Files         929      929              
  Lines      286313   286327      +14     
  Branches    16855    16855              
==========================================
+ Hits       173561   173572      +11     
- Misses     108436   108437       +1     
- Partials     4316     4318       +2

Impacted Files	Coverage Δ
grp/basicprm.gi	87.5% <100%> (-0.15%)	⬇️
hpcgap/src/vars.c	38.02% <0%> (-0.09%)	⬇️
src/read.c	79.71% <0%> (-0.03%)	⬇️
hpcgap/src/funcs.c	48.8% <0%> (ø)	⬆️
src/code.c	86.95% <0%> (+0.08%)	⬆️
src/gap.c	52.99% <0%> (+0.14%)	⬆️

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