[feed] Minor improvements for Feed Binding (#8824)

Signed-off-by: Christoph Weitkamp <github@christophweitkamp.de>

itests/org.openhab.binding.feed.tests/src/main/java/org/openhab/binding/feed/test/FeedHandlerTest.java

@@ -93,7 +93,7 @@ public class FeedHandlerTest extends JavaOSGiTest {
     /**
      * It is updated from mocked {@link StateChangeListener#stateUpdated() }
      */
-    private StringType currentItemState = null;
+    private StringType currentItemState;
 
     // Required services for the test
     private ManagedThingProvider managedThingProvider;

itests/org.openhab.binding.feed.tests/src/main/java/org/openhab/binding/feed/test/SlowTests.java

@@ -12,11 +12,13 @@
  */
 package org.openhab.binding.feed.test;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+
 /**
  * This interface is used to mark tests that take too much time
  *
- * @author Svilen Valkanov
+ * @author Svilen Valkanov - Initial contribution
  */
+@NonNullByDefault
 public interface SlowTests {
-
 }