fix nullness violation in MapDB test (#881)

Signed-off-by: Markus Rathgeb <maggu2810@gmail.com>

bundles/org.openhab.core.storage.mapdb/src/test/java/org/eclipse/smarthome/storage/mapdb/internal/MapDbStorageServiceTest.java

@@ -75,7 +75,7 @@ public class MapDbStorageServiceTest {
             configuration.put(key, value);
         }
 
-        public Object get(String key) {
+        public @Nullable Object get(String key) {
             return configuration.get(key);
         }
     }