realpath: Additional test cases

• Passing NULL should result in EINVAL
• Passing an empty path should result in ENOENT
• Failure with a non-null buffer should leave a partial result. As pointed out in a comment in the test case, this reveals a discrepancy between the documentation and reality.

Approved by: re (cperciva)
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53024

(cherry picked from commit f3386dfeb429faaa30a915a4a422a25e07c8bf39)
(cherry picked from commit 5debbcb6b7a5a25511e8d63142fbc5e8498a8f46)