XmStringEmpty(3) on Linux
The text ad5f23c04d1f, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A compound string function that provides information on the existence of non-zero-length text components "XmStringEmpty" "compound string functions" "XmStringEmpty"