[Bug]: FlinkRunner does not emit watermark with empty source

[je-ik]

What happened?

If unbounded source is empty, watermarks are not sent from the source to downstream operators.

Issue Priority

Priority: 3 (minor)

Issue Components

• [ ] Component: Python SDK
• [ ] Component: Java SDK
• [ ] Component: Go SDK
• [ ] Component: Typescript SDK
• [ ] Component: IO connector
• [ ] Component: Beam YAML
• [ ] Component: Beam examples
• [ ] Component: Beam playground
• [ ] Component: Beam katas
• [ ] Component: Website
• [ ] Component: Spark Runner
• [X] Component: Flink Runner
• [ ] Component: Samza Runner
• [ ] Component: Twister2 Runner
• [ ] Component: Hazelcast Jet Runner
• [ ] Component: Google Cloud Dataflow Runner

[je-ik]

Related to #31085.