Fix NA in language codes

[bambooforest]

@drammock -- this started off as a fix for (which is now fixed):

testthat::test_file("test-phoneme-uniqueness.R")

but then I started fixing the NA language codes in the same branch to handle:

testthat::test_file("test-language-code-validity.R")

this PR closes #280.

i'm still waiting for answers for:

• https://github.com/phoible/dev/issues/289
• https://github.com/phoible/dev/issues/290

but i think we can merge the current updates.

these codes are still failing:

invalid ISOs: "daf" "duj" "wit" "kxl" "krm" "kxu" "drh" "dit" "mwd" "qgu" "bjd" "ggr" "yiy"

so i made an issue here:

https://github.com/phoible/dev/issues/291

they require some digging.