The Emulator returns an empty pre-commit token when a commit is attempted
without a pre-commit token. This is different from not returning any
pre-commit token at all. The check for 'did the Commit return a pre-commit
token?' did not take this into account, which caused commits on the
Emulator that needed to be retried, not to be retried. This again caused
multiple test errors when running on the Emulator, as this would keep a
transaction present on the test database on the Emulator, and the Emulator
only supports one transaction at a time. These test failures went unnoticed,
because the test configuration for the Emulator had pinned the Emulator
version to 1.5.37, which did not support multiplexed sessions. This again
caused the tests to fall back to using regular sessions.

This change fixes the check for whether a pre-commit token was returned by
a Commit. It also unpins the Emulator version for the system tests using
default settings. This ensures that the tests actually use multiplexed
sessions.

See this failed test run from before making these changes (the only change for this test run was to use the latest emulator version, which triggered multiplexed sessions to be used): https://github.com/googleapis/python-spanner/actions/runs/16940453081/job/48007847941?pr=1402