Fix corner case in IsColiftable

[mohamed-barakat]

gap> LoadPackage( "FinSets", false );
true
gap> I := InitialObject( SkeletalFinSets );
|0|
gap> alpha := UniversalMorphismIntoTerminalObject( I );
|0| → |1|
gap> beta := IdentityMorphism( I );
|0| → |0|
gap> IsColiftable( alpha, beta ); ## the answer is wrong
true
gap> wrong := Colift( alpha, beta );
|1| → |0|
gap> IsWellDefined( wrong );
false
gap> Display( wrong );
{ 0 } ⱶ[ 0 ]→ ∅