Fix TypeError when timesteps list has a gap in blending.steps.forecast - #560

Problem

Calling pysteps.blending.steps.forecast() with a sparse timesteps list that has a gap (e.g. [1, 3], skipping 2) raises:

TypeError: 'NoneType' object is not iterable

at self.__state.final_blended_forecast[j].extend(...) inside __blended_nowcast_main_loop.

Root cause

In StepsBlendingNowcaster.__blended_nowcast_main_loop, worker(j) builds final_blended_forecast_single_member inside for t_sub in self.__state.subtimesteps:, but the line that stores it into final_blended_forecast_all_members_one_timestep[j] is indented one level too deep — inside that same for loop instead of after it:

for t_sub in self.__state.subtimesteps:
    if t_sub > 0:
        ...
        final_blended_forecast_single_member = (
            self.__post_process_output(...)
        )

    final_blended_forecast_all_members_one_timestep[j] = (   # <- inside the for loop
        final_blended_forecast_single_member
    )

When a requested timesteps list has a gap, nowcast_utils.binned_timesteps produces an empty bin for the skipped native step, so self.__state.subtimesteps == [] at that outer step. The for t_sub in []: loop body then never runs, so the assignment never happens, and final_blended_forecast_all_members_one_timestep[j] is left at its initial value of None. That later gets passed to .extend(None).

Note the two existing list-timesteps cases in test_blending_steps.py don't actually exercise this: [1, 2, 3] has no gap, and the other [1, 3] case uses zero_radar=True, zero_nwp=True, which takes an early-exit shortcut that skips the main loop entirely.

Fix

Dedent the assignment one level so it runs once per ensemble member after the subtimesteps loop finishes, regardless of whether that loop ran zero, one, or more iterations — matching the intent of "whatever this member produced at this outer step, even if that's nothing."

Testing

• Added a regression test: timesteps=[1, 3] with non-zero radar/NWP precipitation (the two existing conditions needed to hit this bug at once).
• Reproduced the crash directly against the pre-fix code, confirmed the fix resolves it with no other behavior change.
• Full pysteps/tests/test_blending_steps.py suite: 59 passed (58 existing + 1 new).
• black --check passes with no formatting changes needed.