Return immediately if length is 0

Author: MattiasBuelens

index.bs

@@ -9298,6 +9298,7 @@ a reference to the same object that the IDL value represents.
     1.  Let |arrayBuffer| be |bufferSource|.
     1.  Let |offset| be 0.
     1.  Let |length| be |bufferSource|'s [=BufferSource/byte length=].
+    1.  If |length| is 0, then return the empty [=byte sequence=].
     1.  If |bufferSource| is a [=buffer view type=] instance:
         1.  Set |arrayBuffer| to |bufferSource|'s [=underlying buffer=].
         1.  Set |offset| to |bufferSource|'s [=ArrayBufferView/byte offset=].