There is the Filter called
Thumbnail (alt) from the Media Category that is for the Alt text.
After you add the Relationship for the Image field the
Is empty (NULL) Operator will become available. If the Relationship is not present only the string operators will be listed.
Add the Relationship
image from field_media_image and then add the
Thumbnail (alt) Filter with the
Is empty (NULL) Operator.
It will filter out Media items that have the Alt field available in their structure but not filled in (empty), for example images, as opposed to Files that don’t come with Alt fields in their structure.
On the other hand, using the
Is less than 1 Operator/value combination can help in situations when you need to filter out different Media types to show the items that don’t have the Alt field in their structure at all like File types, while Images with empty alt don’t satisfy that condition.