From Koji Content Generator Metadata[1]:
"maven, win, or image: Legacy build type names which appear at this level
instead of inside typeinfo."
=> see, it's legacy
"typeinfo: A map whose entries are the names of the build types used for
this build, which are free form maps containing type-specific information
for this build."
=> struct{} is used for typeinfo.image because the docs says it should contain
"a free form map", null apparently isn't an option.
[1]: https://docs.pagure.org/koji/content_generator_metadata/
As suggested by the Brew maintainers Tomáš Kopeček and Lubomír Sedlář.
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