Explicitly define behavior for undefined and null values

spec/asyncapi.md

@@ -143,13 +143,15 @@ By convention, the AsyncAPI Specification (A2S) file is named `asyncapi.json` or
 
 When traits ([Operation Trait Object](#operationTraitObject) and [Message Trait Object](#messageTraitObject)) are defined, they MUST be merged into the target object according to the following rules:
 
-* Traits are merged with a recursive merge algorithm:
+* Traits are merged with a recursive merge algorithm, similar to [JSON Merge Patch](https://tools.ietf.org/html/rfc7386):
   * Objects are merged recursively
-  * All other types (including arrays) are overwritten
+  * All other types (including arrays) are overwritten, with the following exceptions
+    * If the value of the source object is `undefined` it is not merged
+    * If the value of the source object is `null`, the target object property gets removed or set to `null`
 * The target object that the trait is applied is the most specific and therefore is never overwritten, only extended.
 * The order of the trait objects defines its specificity. Subsequent trait objects in the trait array are more specific.
 * The resulting merge order is the following, with the most specific object to the right:
-  * `merge(trait1, trait2, trait3, ..., targetObject)`
+  * `trait1`, `trait2`, `trait3`, ..., `target object`
 
 ### <a name="schema"></a>Schema