make sure Matrix([]) == Matrix([[]])

constant_sytem must recognize when A is a zero-sized matrix (or
else row_join and col_join need to be modified to allow joins
for a zero-sized matrix and something else, e.g.
Matrix(0,1,[]).row_join(eye(1))).
Merge pull request #2761 from smichr/mat0

make sure Matrix([]) == Matrix([[]])