The viewing_content condition does not work

Issue Summary

This is reproducible on Data Center: No

Steps to Reproduce

1. Create an app that displays a web panel with the `viewing_content` condition. e.g. use the sample app at https://glitch.com/edit/#!/dx-viewing-content-condition?path=connect.json%3A30%3A1 with descriptor URL https://dx-viewing-content-condition.glitch.me/connect.json

Expected Results

The app's panel at location `atl.general` should only appear when the user is viewing a page or blog post.

Actual Results

The panel is always displayed if the condition doesn't have the condition parameter "viewMode": true or false. The panel is never displayed if the condition does have the condition parameter "viewMode": true or false. 

Workaround

None