Crash on index3d_is_empty with NULL argument

[XLegland]

index3d_is_empty(NULL) leads to a crash. We should return if the argument is NULL to avoid crashing

[aduermael]

@XLegland I'm not sure about this. So you would return true? Suggesting that the index is empty? (while in fact NULL) This could lead devs to other errors, not necessarily easier to debug. We could simply add an assert in that function.

[gdevillele]

let's use an assert for now