Update spec/Section 7 -- Response.md

Co-authored-by: Benjie <[email protected]>

spec/Section 7 -- Response.md

@@ -109,7 +109,7 @@ must contain an entry with the key `path` that details the path of the response
 field which experienced the error. This allows clients to identify whether a
 `null` result is intentional or caused by a runtime error.
 
-This field must be a list of path segments starting at the root of the response
+If present, this field must be a list of path segments starting at the root of the response
 and ending with the field associated with the error. Path segments that
 represent fields must be strings, and path segments that represent list indices
 must be 0-indexed integers. If the error happens in an aliased field, the path