[Fix-16789] Fix flink job stop error when process has already exited

[KaiSong-UK]

What is the purpose of the change

fix #16789

When stopping a Flink task through the DolphinScheduler page, if the Flink process has already exited automatically before the kill signal is sent, the pstree command in ProcessUtils.getPidList() throws an exception because the process no longer exists. This causes the kill() method to return false and log an error message, even though the process termination is actually the expected behavior.

Brief changelog

• Modified ProcessUtils.getPidList() to return an empty list when pstree fails (process has already exited), instead of throwing an exception
• Changed the log level from error to warn with a more descriptive message indicating the process may have already exited

Verifying this change

When a Flink task process exits on its own before the stop command is issued, the kill operation will now succeed silently instead of reporting an error.

Does this PR introduce breaking changes?

No.

[SbloodyS]

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