man xargs:
-r Compatibility with GNU xargs. The GNU version of xargs runs the utility argument at least once, even if xargs input is empty, and it supports a -r option to inhibit this
behavior. The FreeBSD version of xargs does not run the utility argument on empty input, but it supports the -r option for command-line compatibility with GNU xargs, but the
-r option does nothing in the FreeBSD version of xargs.