Fix LowerCentralSeriesOfGroup for trivial groups (gap-system#6110)

Fix `LowerCentralSeriesOfGroup` and `NilpotencyClassOfGroup`
for trivial groups to return a list of length 1 resp. the
value 0 (and not a list of length 2 resp. the value 1).

Author: fingolfin

lib/grpperm.gi

@@ -1447,6 +1447,10 @@ InstallMethod( LowerCentralSeriesOfGroup,"for permgrp", true, [ IsPermGroup ], 0
     function( G )
     local  pcgs,  series;
 
+    if IsTrivial(G) then
+        return [G];
+    fi;
+
     if    not DefaultStabChainOptions.tryPcgs
        or HasIsNilpotentGroup( G )  and  not IsNilpotentGroup( G )
        and not (HasIsNilpotentGroup(G) and IsNilpotentGroup(G)) then

tst/testbugfix/2025-09-11-LowerCentralSeriesOfGroup.tst

@@ -0,0 +1,7 @@
+# Fix a bug in LowerCentralSeriesOfGroup for trivial groups
+# See https://github.com/gap-system/gap/issues/6108
+gap> D:=Group(());;
+gap> LowerCentralSeriesOfGroup( D );
+[ Group(()) ]
+gap> NilpotencyClassOfGroup(D);
+0