[R] Bindings for str_replace_na()

[asfimport]

Implement the stringr function str_replace_na() which is useful in combination with str_c().

Reporter: Ian Cook / @ianmcook

Related issues:

• [R][C++] Bindings for paste(), paste0(), str_c() (relates to)

_{Note: This issue was originally created as ARROW-13078. Please see the migration documentation for further details.}

[github-actions]

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