Fixed bug when exterior power is 0-dimensional

[laurentbartholdi]

There's a bug with ExteriorPower:

gap> wedge := ExteriorPower(Rationals^10,11);;
gap> Dimension(wedge);
1

when it should be 0.

[codecov]

Codecov Report

Merging #1872 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1872      +/-   ##
==========================================
+ Coverage   63.49%   63.49%   +<.01%     
==========================================
  Files         952      952              
  Lines      286804   286803       -1     
  Branches    12722    12722              
==========================================
+ Hits       182103   182106       +3     
- Misses     101899   101900       +1     
+ Partials     2802     2797       -5

Impacted Files	Coverage Δ
lib/algrep.gi	57.09% <100%> (+0.06%)	⬆️
src/iostream.c	46.24% <0%> (-0.4%)	⬇️
src/objset.c	82.36% <0%> (-0.24%)	⬇️
src/hpc/traverse.c	78.29% <0%> (-0.09%)	⬇️
src/funcs.c	74.75% <0%> (ø)	⬆️
src/hpc/threadapi.c	34.3% <0%> (+0.09%)	⬆️
src/lists.c	56.71% <0%> (+0.11%)	⬆️
src/stats.c	79.15% <0%> (+0.13%)	⬆️
src/hpc/thread.c	45.84% <0%> (+0.39%)	⬆️

[fingolfin]

This change looks sensible, but could you please add a test case (in a .tst file) for the bug? E.g. the one you describe in the PR