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MINOR: [R] Adapt stringr::str_c mapping for upcoming release (#14296)

Previously, `str_c(sep = NA)` would just emit `NA`, but in the upcoming stringr release, it will error. 

cc @hadley 

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>