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ARROW-17543: [R] Fix bug for NULL type 0-length vectors in array creation

Added the suggested check in ```Array$create()``` from the JIRA issue, that if both the ```x``` and ```type``` are ```NULL``` then the type is manually assigned to ```arrow::null()```.

Added unit tests that captured the bug before I applied the fix and now pass.

Closes #13990 from egillax/detect_and_fix_type_null_vectors

Authored-by: Egill Fridgeirsson <egillax@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>