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.

Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53024