makes bundles friendly to -Ywarn-dead-code

Apparently, the `new Bundle(???).impl` synthetic tree generated as a
macro impl ref for bundles evokes -Ywarn-dead-code warnings.

This pull requests changes `???` to `null` in order not to stress out
the checker. What's in the argument doesn't actually make any difference
anyway.
Merge pull request #3689 from xeno-by/ticket/8523

makes bundles friendly to -Ywarn-dead-code