Ensure proper handling of "partially missing" complex inputs where appropriate

[MichaelChirico]

Follow-up to #5748.

We need to ensure our complex handling behaves "as expected" in the improved world of missingness for complex values in R.

coalesce() in particular seems worthy of further scrutiny (skipped for now in the interest of just getting tests green as quickly as possible):

showz = \(z) cbind(Re = Re(z), Im = Im(z))

showz(fcoalesce(complex(real = NA, imaginary = NA), 0+1i)) # i.e. NA_complex_
#      Re Im
# [1,]  0  1
showz(fcoalesce(complex(real = 0 , imaginary = NA), 0+1i)) # i.e. unchanged
#      Re Im
# [1,]  0 NA
showz(fcoalesce(complex(real = NA, imaginary = 0 ), 0+1i)) # i.e. unchanged
#      Re Im
# [1,] NA  0

If this is deemed problematic we should check handling elsewhere in the codebase too.

[jangorecki]

I think it is good. If complex number is only partially missing then fcoalesce can ignore it.