Correctly mark getDefaultConvention as Nullable

Author: shakuzen

micrometer-observation/src/main/java/io/micrometer/observation/docs/ObservationDocumentation.java

@@ -75,6 +75,7 @@ default String getName() {
      * {@link #getContextualName()}.
      * @return default naming convention
      */
+    @Nullable
     default Class<? extends ObservationConvention<? extends Observation.Context>> getDefaultConvention() {
         return null;
     }