Fail build when inputs are empty

It can easily happen that, for example, a workflow file contains an input version: ${{ matrix.julia-version }} while the strategy matrix only contains a key ${{ matrix.version }}. In that case, we want the action to fail, rather than trying to download julia from an URL that's missing parts and 404ing. We could fall back to the default but that means that builds silently do things differently than they're meant to, which is worse than failing the build.