[R] Implement support for keepNA = FALSE in base::nchar()

[HyukjinKwon]

Describe the enhancement requested

Should address TODO added in commit 81e1fbc (ARROW-17665)

arrow/r/R/dplyr-funcs-string.R

Line 488 in 744f0ec

# TODO: I think there is a fill_null kernel we could use, set null to 2

Component(s)

R