Fix regression in determineTransactionManager

The fix in cec26e9 for SPR-12541 actually introduced a regression
when the interceptor is enabled on a method that does not require any
transaction. In such a case we try to locate the default
PlatformTransactionManager instead of just returning what we have (that
is null).

This commit updates the determineTransactionManager condition again to
take that use case into account again.

Issue: SPR-12541