Add three unit tests to TestStreamAlerts in tests/unit/test_streaming.py covering the edge case where stream_alerts() is called and the monitors list returned by the API is empty.

What changed

• tests/unit/test_streaming.py: three new tests added to TestStreamAlerts:
  • test_stream_alerts_empty_monitors_yields_nothing -- empty list on the first poll yields no alerts
  • test_stream_alerts_empty_monitors_sleep_still_called -- asyncio.sleep is still invoked when monitors is empty (the loop does not exit early)
  • test_stream_alerts_empty_then_monitors_appear_no_transition -- monitors present on the second poll after an empty first poll set the baseline without emitting a transition alert, because there is no prior state to diff against

Why this shape

The three tests each pin a distinct observable behaviour of the empty path: no yield, no early exit, and correct baseline initialisation. Splitting them makes each failure message self-describing. No production code changes are required; the existing implementation already handles this path correctly.

Verification

Acceptance criteria

• stream_alerts() with an empty monitors list correctly yields nothing
• baseline is empty after the first empty poll (observable via the no-transition test on second poll)
• the path has unit test coverage guarding against regressions