BUG: make a fully connected csgraph from an ndarray with no zeros

Should close https://github.com/scipy/scipy/issues/3818.  A problem was that `np.ma.is_masked(x)` returns `False` if `x` is a masked array with no elements masked-out.  The change in this PR might look disruptive but it passes all of the `scipy.sparse` unit tests locally, plus a new one.