fix: Prevent infinite loop for images with no dimensions

This fixes the plugin to prevent it from infinite looping and crashing the browser tab when an image with no dimensions in the node attributes is encountered.

We saw this issue when migrating from an existing image ProseMirror node schema that had no width and height attributes (i.e. they came through to the plugin as null).

With this fix, the images display as expected, and a seamless migration to this plugin is enabled.