Canonicalize construction of Decoupled with no payload (#785)

* Canonicalize construction of empty Decoupled

* Change signature after dev meeting discussion
* Make EmptyBundle private and final

* Add test case for Decoupled with no payload

* Apply suggestions from code review

Co-authored-by: Richard Lin <richard.lin@berkeley.edu>

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Adam Izraelevitz <azidar@gmail.com>
Co-authored-by: Richard Lin <richard.lin@berkeley.edu>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>