RStringVector I/O of `NULL` values should be consistent with R vectors

[swells]

When we pass a RStringVector from the client with all NULL values we get incorrect and inconsistent results:

1. R console output produces boolean/logical True - incorrect
2. Return type in the calling client is RDataNA which is not a vector with NULL values - incorrect

Results should be consistent with R, for example:

> x <- c(NULL, NULL, NULL, NULL)
> x
NULL

> y <- c(NULL, NULL, "foo", NULL)
> y
[1] "foo"

Reproduce:

List<String> v = new ArrayList<String>();
v.add(null);
v.add(null);
v.add(null);
v.add(null);
RDataFactory.createStringVector("x", v);

List<String> v = new ArrayList<String>();
v.add(null);
v.add(null);
v.add("foo");
v.add(null);
RDataFactory.createStringVector("y", v);

View response. It should be consistent with R vectors.