Draft: pmb.helpers.package.get: fix !ret + !ret_repo case

(I thought this was important to fix pmaports!5619, but it isn't - making the MR for further reference / future iterations without the hack.)

"ret" gets initialized as {} at the beginning of the function. Fix the code path where 'ret_repo = pmb.parse.apkindex.package' may return None, in which case we should not overwrite "ret" with None.

Otherwise we suddenly change the datatype from dict to None and subsequent code fails.

Fixing this uncovered another bug, the arch compare doesn't work because of invalid datatype (#2455 (closed)).