Don't pass values on to labkit that weren't explicityly
defined. Otherwise, labkit will unintentionally overwrite any existing
values with nil.

Consider the following example:

  ApplicationContext.with_context(user: <User "jane-doe">) do
    ApplicationContext.with_context(
      project: <Project "gitlab-org/gitlab">
    ) do
      # The inner context lives here
    end
  end

We want the inner context for Labkit to contain this hash:

  {
    user: "jane-doe",
    project: "gitlab-org/gitlab",
    root_namespace: "gitlab-org"
  }

But before this change, we'd overwrite the `user` value when creating
the inner context, even though the inner context did not contain any
information that should override the user info.