PipelineTest.testEmptyPipeline breaks on Flink runner

```

org.apache.beam.sdk.Pipeline$PipelineExecutionException
:
java.lang.IllegalStateException: No operators
defined in streaming topology. Cannot execute.
Open stacktrace
Caused by:
java.lang.IllegalStateException
:

No operators defined in streaming topology. Cannot execute.
Close stacktrace
at org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.getStreamGraph(StreamExecutionEnvironment.java:1534)
at
org.apache.flink.streaming.api.environment.LocalStreamEnvironment.execute(LocalStreamEnvironment.java:89)
at
org.apache.beam.runners.flink.FlinkPipelineExecutionEnvironment.executePipeline(FlinkPipelineExecutionEnvironment.java:141)
at
org.apache.beam.runners.flink.FlinkRunner.run(FlinkRunner.java:113)

```

Imported from Jira [BEAM-7325](https://issues.apache.org/jira/browse/BEAM-7325). Original Jira may contain additional context.
Reported by: iemejia.

Start with PipelineTest.testEmptyPipeline and reproduce the failure using the Flink runner. Read the reported stack-trace path through FlinkRunner.run and FlinkPipelineExecutionEnvironment.executePipeline. Done means the empty-pipeline test no longer fails with the “No operators defined in streaming topology” exception.

Written by the indexing model from the issue text.