BUG: FeatureAgglomeration: error for n_samples = 0

Trivial fix for trivial bug. Can I haz review, plz.

Currently, the FeatureAgglomeration code does not raises an error when it is given no samples (a matrix of dimensions (0, n_features), but creates a meaningless clustering that only reflects the connectivity matrix.

@schwarty has a nasty bug in his codebase because of that.