move unspecifiedAttr check to checkAttr

move unspecifiedAttr check to checkAttr

It just so happens that everywhere that checks attrs other than
annex.largefiles parses the value further, and failed to parse
unspecifiedAttr in a way that behaved the same as if nothing was set. So
this is not a bug fix or behavior change. What it does so is prevent
future uses of checkAttr from needing to remember to handle checking for
this edge case.

Sponsored-by: Dartmouth College's DANDI project