Merge pull request #357 from HubSpot/optional-url

Make url optional

Author: pmusgrave

slack-base/src/main/java/com/hubspot/slack/client/models/blocks/ImageIF.java

@@ -22,7 +22,7 @@ default String getType() {
   }
 
   @Value.Parameter
-  String getImageUrl();
+  Optional<String> getImageUrl();
 
   @Value.Parameter
   String getAltText();