Deprecate NothingHappened in favor of NoReaction

We cannot say "nothing happened" in response to an event, because something indeed happened. So, `NoReaction` is a better way of describing the case when we do not want to react on an event.

Author: alexander-yevsyukov

server/src/main/kotlin/io/spine/server/event/Policy.kt

@@ -66,8 +66,7 @@ import io.spine.string.joinBackticked
  * from your Kotlin method, or `Just.nothing()` from Java.
  *
  * If you need to avoid the naming collision with [kotlin.Nothing], consider using
- * type aliases [NothingHappened][io.spine.server.model.NothingHappened] or
- * [NoReaction][io.spine.server.model.NoReaction].
+ * the [NoReaction][io.spine.server.model.NoReaction] type alias.
  *
  * ### Returning one event
  * To return one event, declare `Just<MyEvent>` as the return type of the [whenever] method.
@@ -91,7 +90,6 @@ import io.spine.string.joinBackticked
  * @see [io.spine.server.tuple.Quartet]
  * @see [io.spine.server.tuple.Quintet]
  * @see [io.spine.server.model.Nothing]
- * @see [io.spine.server.model.NothingHappened]
  * @see [io.spine.server.model.NoReaction]
  */
 public abstract class Policy<E : EventMessage> : AbstractEventReactor(), WithLogging {

server/src/main/kotlin/io/spine/server/model/EventExts.kt

@@ -30,6 +30,7 @@ package io.spine.server.model
  * The alias for cases of naming collisions between
  * [io.spine.server.model.Nothing] and [kotlin.Nothing].
  */
+@Deprecated(message = "Please use `NoReaction` instead", replaceWith = ReplaceWith("NoReaction"))
 public typealias NothingHappened = Nothing
 
 /**