The docs clearly state

Like gl_Position. When not present, a default assignment statement is generated automatically using MODELVIEWPROJECTION_MATRIX and VERTEX. This is why an empty MAIN() is functional, and in most cases there will be no need to assign a custom value to it.

But an empty MAIN() in an Unshaded vertex shader does not display anything.

Setting POSITION in MAIN() shows the object as expected.
POSITION = MODELVIEWPROJECTION_MATRIX * vec4(VERTEX, 1.0);