The problem is that USE_XORG may be defined by other Uses modules, such as gl, but still empty.

For example

USES=gl
USE_GL=glut

leads to USE_XORG set via (in Uses/gl.mk):

USE_XORG+=  ${_GL_${_component}_USE_XORG}

to empty value, as _GL_glut_USE_XORG is not defined

This reproduces on graphics/devil when X11 option is off.