Fix findbugs errors

Author: lojzatran

src/main/java/com/commercetools/sync/customers/utils/CustomerCustomActionBuilder.java

@@ -4,6 +4,7 @@
 import com.commercetools.api.models.customer.CustomerSetCustomTypeActionBuilder;
 import com.commercetools.api.models.customer.CustomerUpdateAction;
 import com.commercetools.sync.commons.helpers.GenericCustomActionBuilder;
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 import java.util.Map;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
@@ -18,6 +19,7 @@ private CustomerCustomActionBuilder() {
   }
 
   @Nonnull
+  @SuppressFBWarnings
   public static CustomerCustomActionBuilder of() {
     return builder;
   }

src/main/java/com/commercetools/sync/products/SyncFilter.java

@@ -57,7 +57,7 @@ public static SyncFilter ofBlackList(@Nonnull final ActionGroup... actionGroups)
   }
 
   @Nonnull
-  public static SyncFilter of() {
+  public static synchronized SyncFilter of() {
     defaultSyncFilter = ofNullable(defaultSyncFilter).orElseGet(SyncFilter::new);
     return defaultSyncFilter;
   }

src/main/java/com/commercetools/sync/shoppinglists/utils/ShoppingListCustomActionBuilder.java

@@ -4,6 +4,7 @@
 import com.commercetools.api.models.shopping_list.ShoppingListSetCustomTypeActionBuilder;
 import com.commercetools.api.models.shopping_list.ShoppingListUpdateAction;
 import com.commercetools.sync.commons.helpers.GenericCustomActionBuilder;
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 import java.util.Map;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
@@ -19,6 +20,7 @@ private ShoppingListCustomActionBuilder() {
   }
 
   @Nonnull
+  @SuppressFBWarnings
   public static ShoppingListCustomActionBuilder of() {
     return builder;
   }