Revert matrix changes

Author: lgoettgens

src/MatRing.jl

@@ -441,6 +441,5 @@ Return parent object corresponding to the ring of $n\times n$ matrices over
 the ring $R$.
 """
 function matrix_ring(R::NCRing, n::Int)
-   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
-   return Generic.matrix_ring(R, n)
+   Generic.matrix_ring(R, n)
 end

src/Matrix.jl

@@ -6633,7 +6633,6 @@ randmat_with_rank(S::MatSpace{T}, rank::Int, v...) where {T <: RingElement} =
 Constructs the matrix over $R$ with entries as in `arr`.
 """
 function matrix(R::NCRing, arr::AbstractMatrix{T}) where {T}
-   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
    if elem_type(R) === T && all(e -> parent(e) === R, arr)
       z = Generic.MatSpaceElem{elem_type(R)}(R, arr)
       return z
@@ -6644,7 +6643,6 @@ function matrix(R::NCRing, arr::AbstractMatrix{T}) where {T}
 end
 
 function matrix(R::NCRing, arr::MatElem)
-   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
    return map_entries(R, arr)
 end
 
@@ -6683,7 +6681,6 @@ Constructs the $r \times c$ matrix over $R$, where the entries are taken
 row-wise from `arr`.
 """
 function matrix(R::NCRing, r::Int, c::Int, arr::AbstractVecOrMat{T}) where T
-   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
    _check_dim(r, c, arr)
    ndims(arr) == 2 && return matrix(R, arr)
    if elem_type(R) === T && all(e -> parent(e) === R, arr)
@@ -7065,7 +7062,6 @@ the ring $R$.
 function matrix_space(R::NCRing, r::Int, c::Int; cached::Bool = true)
   # TODO: the 'cached' argument is ignored and mainly here for backwards compatibility
   # (and perhaps future compatibility, in case we need it again)
-  @req !is_trivial(R) "Zero rings are currently not supported as base ring."
   (r < 0 || c < 0) && error("Dimensions must be non-negative")
   T = elem_type(R)
   return MatSpace{T}(R, r, c)