Improve JdbcExecutor traversal by safely handling missing or multiple consumers

[mohit-devlogs]

This change improves the robustness of JdbcExecutor by relaxing the assumption that the retrieval of consumers from the output channel is a singleton.
In the previous code, the function findJdbcExecutionOperatorTaskInStage() in the class WayangJdbcExecutor used the function WayangCollections.getSingle() for the retrieval of consumers from the output channel. The function throws an IllegalArgumentException if the list of consumers is empty or if there are more than one consumers in the list.

Changes:

• More robust code is now used in place of WayangCollections.getSingle()
• Return null if no consumer exists (i.e., it is the last in the pipeline)
• Throw a Wayang exception if there are more than one consumers

This prevents crashes like this:
IllegalArgumentException: [] is not a singleton

Testing:

• Compilation checked
• Tested with:
  mvn -pl wayang-platforms/wayang-generic-jdbc test