Throw exception if contentResolver.query() returns null

[artem-zinnatullin]

At the moment we return empty MatrixCursor if original call to contentResolver.query() returns null, but in fact if ContentProvider returns null, it means that this query can not be processed because of some error/exception.

So we should throw it as exception.

@nikitin-da and others thoughts?

See: http://stackoverflow.com/questions/13080540/what-causes-androids-contentresolver-query-to-return-null

[nikitin-da]

This decision was made using only sources, without docs. Can it be platform specific? If it's always error state, it's nice idea to follow non-null approach - throw runtime exception.

[artem-zinnatullin]

Documentation for ContentProvider says that query() can return Cursor or null. In my opinion, if there is no data for this query (eg table is empty) ContentProvider should return empty cursor, but if such query does not have mapping (eg no such table for uri and so on) it should return null, which is exceptional case for us and users of StorIO, I guess.

[artem-zinnatullin]

Closed by #451.