[FLINK-39285][table] Ignore project rel node generated by FlinkFilterJoinRule in JoinToMultiJoinRule

[ldadima]

What is the purpose of the change

Consider the result of filter pushdown rules in the MultiJoin rule, according FLINK-39285

Verifying this change

Run MultiJoinTest#testThreeWayComplexJoinWithConditionsRelPlan

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): ( no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? ( no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• ce4a168 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build