Based on the definition of SVG1.1 and SVG2:

The EBNF allows the path data string in the d property to be empty. An empty path data string disables rendering of the path. Rendering is also disabled when the d property has the value none.

So an empty path string is valid. And then what is the expected computed value for this case?

offset-path: path(' ');

If someone calls getComputedStyle(div).offsetPath, should we return path('') or none for offset-path? It seems Blinks return none for now. It looks like the spec doesn't mention this, anyway.