Fix ZeroDivisionError at initial invocation of monitoring_info when there is no work yet

[Abacn]

It is found running a streaming pipeline in Python, there is an error at the beginning:

Traceback (most recent call last):
  File "...beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 310, in _execute
    response = task()
  File "...beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 347, in <lambda>
    lambda: self.create_worker().do_instruction(request), request)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "...beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 671, in do_instruction
    return getattr(self, request_type)(
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        getattr(request, request_type), request.instruction_id)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...beam/sdks/python/apache_beam/runners/worker/sdk_worker.py", line 762, in process_bundle_progress
    monitoring_infos = processor.monitoring_infos()
  File "...beam/sdks/python/apache_beam/runners/worker/bundle_processor.py", line 1441, in monitoring_infos
    op.monitoring_infos(transform_id, dict(tag_to_pcollection_id)))
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "apache_beam/runners/worker/operations.py", line 1111, in apache_beam.runners.worker.operations.SdfProcessSizedElements.monitoring_infos
    def monitoring_infos(self, transform_id, tag_to_pcollection_id):
  File "apache_beam/runners/worker/operations.py", line 1116, in apache_beam.runners.worker.operations.SdfProcessSizedElements.monitoring_infos
    with self.lock:
  File "apache_beam/runners/worker/operations.py", line 1125, in apache_beam.runners.worker.operations.SdfProcessSizedElements.monitoring_infos
    completed = current_element_progress.fraction_completed
  File "...beam/sdks/python/apache_beam/io/iobase.py", line 1720, in fraction_completed
    return float(self._completed) / self.total_work
           ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
ZeroDivisionError: float division by zero

This is because a RestrictionProgress {'fraction': None, 'remaining': 0.0, 'completed': 0} is initialized at

  File "apache_beam/runners/worker/operations.py", line 1116, in apache_beam.runners.worker.operations.SdfProcessSizedElements.monitoring_infos
    with self.lock:
  File "apache_beam/runners/worker/operations.py", line 1119, in apache_beam.runners.worker.operations.SdfProcessSizedElements.monitoring_infos
    current_element_progress = self.current_element_progress()
  File "apache_beam/runners/worker/operations.py", line 1101, in apache_beam.runners.worker.operations.SdfProcessSizedElements.current_element_progress
    with self.lock:
  File "apache_beam/runners/worker/operations.py", line 1106, in apache_beam.runners.worker.operations.SdfProcessSizedElements.current_element_progress
    return progress.with_completed(
  File "...beam/sdks/python/apache_beam/io/iobase.py", line 1733, in with_completed
    return RestrictionProgress(
        fraction=self._fraction, remaining=self._remaining, completed=completed)

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[github-actions]

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R: @tvalentyn for label python.

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[tvalentyn]

Thanks!