allow for empty SparseSeries SparsePanel constructors
Closes #9272.

Note that my solution is to provide a default `data=[]` for SparseSeries and `frames={}` for SparsePanel. This differs from the dense implementations that have `None` defaults that are then changed to the above. Doesn't seem like a big deal to me, but I can do that here if needed.

pls add a release note as well (call this a bug fix).
 I'm guessing that this was buried under more important things, but let me know if you'd like any additional changes.
 merged via https://github.com/pydata/pandas/commit/391f46af129d214056ec9109caa4c53b7dddfb04

thanks!
