- Use empty array when `git credential-osxkeychain` lookup fails to
cache and avoid rerunning it when there's no valid results.
- Redirect `stderr` to avoid printing errors when there's a failure
or no `git credential-osxkeychain` installed.
ClosesHomebrew/homebrew#49954.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>