Add language to account for empty transform in XRReferenceSpaceEvent

index.bs

@@ -2510,7 +2510,7 @@ dictionary XRReferenceSpaceEventInit : EventInit {
 
 The <dfn attribute for="XRReferenceSpaceEvent">referenceSpace</dfn> attribute indicates the {{XRReferenceSpace}} that generated this event.
 
-The <dfn attribute for="XRReferenceSpaceEvent">transform</dfn> attribute describes the post-event position and orientation of the {{XRReferenceSpaceEvent/referenceSpace}}'s [=native origin=] in the pre-event coordinate system.
+The optional <dfn attribute for="XRReferenceSpaceEvent">transform</dfn> attribute describes the post-event position and orientation of the {{XRReferenceSpaceEvent/referenceSpace}}'s [=native origin=] in the pre-event coordinate system. This attribute MAY be `null` if the {{XRSystem}} can't determine the delta between the old and the new coordinate system.
 
 Event Types {#event-types}
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