[6.1] Home directory for non-existent user should not fall back to /var/empty or %ALLUSERSPROFILE%

[jmschonfeld]

• Explanation: Requesting the home directory for a non-existent user now returns nil as it used to instead of /var/empty or %ALLUSERSPROFILE%. The current user's home directory still falls back to /var/empty/%ALLUSERSPROFILE% as it has previously.
• Scope: Impacts requesting the home directory for a username that does not exist across all platforms
• Original PRs: Home directory for non-existent user should not fall back to /var/empty or %ALLUSERSPROFILE% #1072
• Risk: Low-Medium - this does change behavior, however it restores swift 5.10 behavior, this is heavily tested by unit tests, and I don't expect this behavior to be fairly load bearing
• Testing: local testing, swift-ci testing
• Reviewers: @jrflat @kperryua

[jmschonfeld]

@swift-ci please test