Prior to this commit, if you wanted to use modifiers such as
`:exclusions` or `:classifier` for a dependency whose version you
were managing with `:managed-dependencies`, you would need to
explicitly pass a `nil` as the version string in the dependency
tuple. This commit adds some logic to coerce the vectors before
they are processed, so that if the version string is simply
omitted instead of being set to `nil`, the `nil` will be implicitly
inserted and things will continue to work as before.
This provides a slightly nicer and more intuitive UX for the
managed-dependencies feature.
5bd091a2bf