ENH sparse: Allow sparse matrices to be size 0.
TST sparse: Test arithmetic & slicing with size zero sparse matrices.
TST: sparse: more zero-size matrix tests
Merge pull request #2755 from cowlicks/size-0-matrices

ENH: sparse: Size zero sparse matrices

This allows sparse matrices to be size zero. Meaning one or both of their shape
components is == 0.  This is needed because fancy indexing and other weird edge
cases occasionally need to return a size-0 sparse matrix.