Instead, the comma is emitted when there is indeed an extra argument.

This change was initially submitted in #3147 and is needed when the
extra argument may vary for a given symbol. More specifically, when
a method symbol may be shared by several instance symbols, it becomes
really difficult to find a spot where that extra comma might be added.

Removing it from the extra argument itself makes this easier once we
reach the point where method symbols are unique (per object) instead
of repeated (per instance).