TL;DR
The default value is /, as in
<key>WorkingDirectory</key>
<string>/</string>
Explanation
The WorkingDirectory in a launchd .plist is an optional key used to specify a directory to chdir(2) to before running the job. If this key is not present, then the root / directory is used.
I verified this by running a shell script as both a Launch Agent and a Launch Daemon that, when run, appended a directory listing to a text file. In all cases, without the WorkingDirectory key, the output was the root / directory of the startup disk.
Depending on whether or not it was run as root or the current user the output as root also contained the hidden directories even though that was not explicitly set to output by the script.
As a side note, the PATH passed to the shell script when run as both a Launch Agent and Launch Daemon was:
/usr/bin:/bin:/usr/sbin:/sbin
Note that this was tested under OS X 10.8.5.
/is now read-only on macOS Catalina, so it may be worth settingWorkingDirectoryto something like/tmpnow, depending on what you need. – jjj Sep 06 '19 at 13:19/and if you want to change it, then set theWorkingDirectorykey to what you want it to be. Also, you have to use the actual path, not variables. – user3439894 Jul 05 '20 at 02:39