Fix dag file processor hertbeat to run only if necessary (#30899)

The standalone file processor as of #30278 introduced accidentally
an artifficial delay between dag processing by adding heartbeat
but missing to set "only_if_necessary" flag to True.

If your dag file processing has been fast (faster than the
scheduler job_heartbeat_sec) this introduced unnecessary pause
between the next dag file processor loop (up until the time
passed), it also introduced inflation of the
dag_processing_last_duration metrics (it would always show minimum
job_heartbeat_sec)

Adding "only_if_necessary" flag fixes the problem.

Fixes: #30593
Fixes: #30884
(cherry picked from commit 00ab45f)

Author: potiuk

airflow/jobs/dag_processor_job_runner.py

@@ -57,7 +57,7 @@ def __init__(
         self.processor.heartbeat = lambda: perform_heartbeat(
             job=self.job,
             heartbeat_callback=empty_callback,
-            only_if_necessary=False,
+            only_if_necessary=True,
         )
 
     def _execute(self) -> int | None: