`list_index_named_values` enable correctly finding is.NA if named_values = NA

[snystrom]

Currently list_index_named_values will throw a warning & ignore any named_values set to NA. This it to prevent subsetting to NAs. I could do better here and fix this to work to find values with NA's, which isn't unreasonable to want to find.

[snystrom]

Is this needed? If the list is passed to cmd_args_to_flags() NA's are dropped. But maybe someone will want to have conditional behavior with NA value? Need to think more about this.