When listing the page history, the earliest revision does not have a diff to the previous version (the link with the 'prev' text) because there is no previous version.
For complete page histories, this is correct. But when searching within the page history (eg using the tag filter) it should provide links for diffs if there is a previous revision that is not included in the results.
The same goes for the diff to current version (the 'cur' link) for the most recent revision returned by the search.