Fix crash on undefined image url

[mkevins]

Description

This PR updates the Gutenberg reference to bring in the changes in this companion PR: WordPress/gutenberg#15532.

This PR adds a check to ensure attributes.url exists before attempting to call indexof. Currently, there are scenarios which can result in attributes.id being truthy, while attributes.url is null or undefined for an image block.

When an image is cancelled before it is finished uploading, the url is set to null, but the id of the block remains. The next time the post is edited, the local draft will contain the state which triggers the crash.

Testing steps:

This was tested using the steps here: wordpress-mobile/WordPress-Android#9768 (comment)

Types of changes

This is a bug fix for the crash discovered here: wordpress-mobile/WordPress-Android#9768

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

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[hypest]

Hey @mkevins , I guess the v1.4.0 part of the branch name should be bumped to 1.4.1 since 1.4.0 is already deployed. One way to address this without creating a new PR is to make this PR target a hotfix/1.4.1 branch instead of master. If and when we decide to deploy that as a "full" hotfix, we can open a PR from hotfix/1.4.1 to master to track it. How does that sound?

[mkevins]

I've created the hotfix branch with the correct version, and now merged this branch to that one. I have opened a PR on that branch targeting master.