Trac #33107: return immutable trivial Cholesky factors.

We usually return an immutable factor from cholesky(), but in the
special trivial case the factor was mutable (a copy of the input
matrix). Here we make it immutable in both the superclass method
and in the RDF/CDF subclass.

src/sage/matrix/matrix2.pyx

@@ -12626,6 +12626,9 @@ cdef class Matrix(Matrix1):
             sage: all( matrix(R,[]).cholesky() == matrix(R,[])
             ....:      for R in (RR,CC,RDF,CDF,ZZ,QQ,AA,QQbar) )
             True
+            sage: all( matrix(R,[]).cholesky().is_immutable()
+            ....:      for R in (RR,CC,RDF,CDF,ZZ,QQ,AA,QQbar) )
+            True
 
         """
         cdef Matrix C # output matrix
@@ -12642,7 +12645,9 @@ cdef class Matrix(Matrix1):
             # The trivial matrix has a trivial cholesky decomposition.
             # We special-case this after is_hermitian() to ensure that
             # the matrix is square.
-            return self
+            C = self.__copy__()
+            C.set_immutable()
+            return C
 
         # Use classical=True to ensure that we don't get a permuted L.
         cdef Matrix L  # block_ldlt() results

src/sage/matrix/matrix_double_dense.pyx

@@ -3746,7 +3746,9 @@ cdef class Matrix_double_dense(Matrix_dense):
             raise ValueError(msg.format(self.nrows(), self.ncols()))
         if self._nrows == 0:   # special case
             self.cache(cache_posdef, True)
-            return self.__copy__()
+            L = self.__copy__()
+            L.set_immutable()
+            return L
 
         L = self.fetch(cache_cholesky)
         if L is None: