Replace instances of Size(g)=1 by IsTrivial(g)

Note that there are immediate methods which set IsTrivial suitably as
soon as the Size is known, hence these changes do not break any existing
use case. However, there are examples where computing Size is
infeasible, but computing IsTrivial is possible, sometimes even trivial.

lib/grp.gi

@@ -91,7 +91,7 @@ InstallMethod( MinimalGeneratingSet,"cyclic groups",true,
     RankFilter(IsFinite and IsPcGroup),
 function ( G )
 local g;
-  if Size(G)=1 then return [];fi;
+  if IsTrivial(G) then return []; fi;
   g:=Product(IndependentGeneratorsOfAbelianGroup(G),One(G));
   Assert( 1, Index(G,Subgroup(G,[g])) = 1 );
   return [g];
@@ -411,9 +411,9 @@ InstallMethod( IsNilpotentGroup,
 #M  IsPerfectGroup( <G> ) . . . . . . . . . . . .  test if a group is perfect
 ##
 InstallImmediateMethod( IsPerfectGroup,
-    IsGroup and IsSolvableGroup and HasSize,
+    IsSolvableGroup and HasIsTrivial,
     0,
-    grp -> Size( grp ) = 1 );
+    IsTrivial );
 
 InstallMethod( IsPerfectGroup,
     "method for finite groups",