Add unit test for erroneous empty rfile

**Is your feature request related to a problem? Please describe.**
Add a Unit test for checking a valid rfile index entry that points to a datablock that is expected to be non-empty, but returns no data for the block.

Ideally this should throw an exception and interrupt any actions that were dependent on the datablock contents.

Start by locating the existing RFile unit tests and the validation path for an RFile index entry that references a datablock. Create the empty-datablock case described in the issue and verify that it throws an exception and interrupts actions depending on the datablock contents.

Written by the indexing model from the issue text.