#470: simplify nullability handling with Optional

Author: mmerdes

oshdb-api-ignite/src/main/java/org/heigit/ohsome/oshdb/api/db/OSHDBIgnite.java

@@ -207,10 +207,7 @@ public OSHDBIgnite onClose(IgniteRunnable action) {
    * @return the currently set onClose callback
    */
   public Optional<IgniteRunnable> onClose() {
-    if (this.onCloseCallback == null) {
-      return Optional.empty();
-    } else {
-      return Optional.of(this.onCloseCallback);
-    }
+    return Optional.ofNullable(this.onCloseCallback);
   }
+
 }