workaround for IsBijective test

(If GAP is started without packages then something is missing.
This change is just a workaround, one can still run into problems.)

lib/gpfpiso.gi

@@ -912,10 +912,14 @@ InstallMethod( IsomorphismFpGroupByGeneratorsNC,
                "for trivial group",
                [ IsGroup, IsList and IsEmpty, IsString ],
 function( G, emptygens, name )
+    local hom;
+
     if not IsTrivial( G ) then
       Error( "<emptygens> does not generate <G>" );
     fi;
-    return GroupHomomorphismByImagesNC( G, FreeGroup( 0 ), [], [] );
+    hom:= GroupHomomorphismByImagesNC( G, FreeGroup( 0 ), [], [] );
+    SetIsBijective( hom, true );
+    return hom;
 end );