Fix Python 2 bindings for findContours

The sanity check prevents Python 2 from calling findContours, because the
Python 2 bindings pass an empty matrix for `_contours`. If `_contours` is not
empty, the channel and depth check should determine whether the matrix is of
the correct type and size.

Note: the Python 2 bindings passes a matrix of type `STD_VECTOR_MAT` to
parameters of the type `OutputArrayOfArrays`.
Merge pull request #3006 from smvv:fix_findContours_for_python2