Hello,

Running apt satisfy without specifying any relationships to satisfy causes it to gripe about not knowing what packages to satisfy *build dependencies* for:
$ apt satisfy
Error: Must specify at least one package to check builddeps for

Obviously 'apt satisfy' has nothing really to do with ensuring build dependencies are met and, for example, works on binary packages instead of source packages. I assume this is just an artifact of a similar code path being taken.
A hacky but elegant way to fix this, in my opinion, is maybe to let 'apt satisfy' succeed when no relationships are specified: if no additional constraints are specified by the user, then all requests are trivially satisfied. This isn't unusual for APT tools: 'apt install' and such are already idempotent and allow specifying an arbitrary number of packages, including none at all.