fix logic to return True when the date_retrieved file is empty

[hyphaltip]

need to also return true from this test function if the date_retrieval file is empty (which is default after an install).

[AstrobioMike]

thank you so much, Jason! Super coincidentally, i just saw this and fixed it yesterday https://github.com/AstrobioMike/GToTree/commit/9eb248ad5a54563370978d3575727eb63ad93483, haha

Just haven't updated the release or conda yet