Fix jq bugs when version is null (like removed RCs) and drop RC

[yosifkit]

update.sh would correctly remove the old RC now that stable is newer, but generate-stackbrew-library.sh would fail

$ ./generate-stackbrew-library.sh rc
jq: error (at versions.json:136): Cannot iterate over null (null)

Once that was fixed (in the getArches function), the map_values(select(.)) had to be swapped to before to_entries in order fix the next error (the select wasn't removing the null). Since map_values is object key order preserving this is safe.

jq: error (at versions.json:136): split input and separator must be strings