Add migration info for missing values in script

Relates to #30975

docs/reference/migration/migrate_7_0/scripting.asciidoc

@@ -14,6 +14,15 @@ now been removed. Instead, use `.value` on `date` fields, or explicitly
 parse `long` fields into a date object using
 `Instance.ofEpochMillis(doc["myfield"].value)`.
 
+[float]
+==== Accessing missing document values will throw an error
+`doc['field'].value` will throw an exception if
+the document is missing a value for the field `field`.
+
+To check if a document is missing a value, you can use
+`doc['field'].size() == 0`.
+
+
 [float]
 ==== Script errors will return as `400` error codes