Add n/a alias to valueless searches for grade: and rpos:

[kintopp]

rpos: and grade: already work as valueless searches but not with "n/a" standing in as no value (as is the case with Onyomi and Kunyomi). e.g. https://app.kanjialive.com/search/advanced?rpos= gives you all kanji without an explicit radical position classification but https://app.kanjialive.com/search/advanced?rpos=n/a gives zero results.