In HTML ##<img alt="">## is indicating a decorative image, or duplicating textual content, which shall not be mentioned by screenreaders, or not to be described by other reasons.
In wikitext to achieve image transclusion, this is indicated by the following syntax:
[[File:example.svg|alt=|eye-catcher]]
However, many authors regard this as pointless and do remove the ##alt=## which includes the existence of this image again into the aural story.
By definition of
alt=-
it should be made more obvious that a value is assigned, and that the presence of this attribute is intended.
The ##-## is no meanningful story in any language on this planet.
* Therefore a system message consisting of ##-## only is interpreted by MediaWiki as deliberately unset textual message.
* If the parser encounters an ##alt=-##
# the output ##<img alt="">## shall be produced.
BTW, there should be a separate Phabricator trail for ##alt=## image descriptions (even video).