Correctly handle the case of no matrices in the MatrixExpr postprocessors
Correctly handle the case of no matrices in the MatrixExpr postprocessors

For some reason items get cached in the post-processor and it causes them to
be called on Adds and Muls that don't have any top-level MatrixExprs.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy