Don't pre-calc inserted primary key if no getter

Don't pre-calc inserted primary key if no getter

the _setup_ins_pk_from_empty() method provides a placeholder
result for inserted_primary_key_rows that is not typically
going to be invoked.  For an executemany (or even execute)
that isn't already stating it wants primary key values
up front, defer this computation until explicitly requested.

Change-Id: I6295eafbccc96a0422b9cac34e79db7924c702ca
References: https://github.com/sqlalchemy/sqlalchemy/discussions/5893#discussioncomment-356155