code cleanup in a method for DecideZero

for [ IsRingElement, IsHomalgMatrix ]
code cleanup in a method for DecideZero

for [ IsHomalgMatrix, IsHomalgMatrix ], 1001
DecideZero( M, rel ) now checks IsEmptyMatrix( M )
code cleanup in a method for DecideZero

for [ IsHomalgMatrix, IsList ]
code cleanup in a method for DecideZero

for [ IsHomalgMatrix, IsHomalgRingRelations ]
code simplification in constructor method /

for residue class rings