Empty background image property behaving incorrectly

[RyanCoulsonCA]

Description

If a slide is given an empty background image property (i.e., "backgroundImage": ""), the string will be taken literally and the app will set the background image source to the empty string which results in all kinds of funkiness.

Environment

• Browser (if applicable): Any

Screenshots

Expected behavior

If the property is set to an empty string, we should assume the product author does not want a background image and display the white background.

Actual behavior

The intentionally-displayed background image will appear over this slide, but there will also be no white box around the text or content.

Possible solution

This is already fixed in #488. Creating this issue for tracking purposes.