Previously, when inserting an image from a URL, leaving the `alt`
field blank in the media modal would result in an image being
inserted into the editor without an `alt` attribute, rather than
an empty `alt`. This happened because the `props.type` would not
get set in `wp.media.string.props()` — because `attachment` is
undefined in this case — causing the image fallbacks to get
skipped.
This fixes the issue by explicitly setting `props.type` to 'image'
in `wp.media.string.image()` before filling out the rest of the
properties.
Props ambrosey, dabnpits.
Fixes#36735.
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