[C++] Add option to coalesce kernel to treat NaN as null

[asfimport]

Currently the coalesce kernel treats NaN as a non-null value. Consider adding a boolean option to this kernel that could be used to make it treat NaN values the same as null values. This would allow the code added in ARROW-13201 to be much simplified.

Reporter: Ian Cook / @ianmcook

Related issues:

• [R] Add binding for coalesce() (relates to)
• [C++] Add option to is_null kernel to return true on NaN (relates to)

_{Note: This issue was originally created as ARROW-13389. Please see the migration documentation for further details.}