Unhandled error in cylc play --startcp

[wxtim]

Found whilst checking CLI in #6213 :

Description

If a single Cycle workflow has an inital cycle point set either by CLI or config and a --startcp set the by the CLI the workflow shuts down with an unhandled error.

I can't imagine that this is a reasonable set of things to do, and I wouldn't expect it to do anything (R1 being the ICP, not Start CP), but it shouldn't do this:

ERROR - min() arg is an empty sequence
    Traceback (most recent call last):
      File "/net/home/h02/tpilling/metomi/cylc-flow/cylc/flow/scheduler.py", line 741, in start
        await self.configure(params)
      File "/net/home/h02/tpilling/metomi/cylc-flow/cylc/flow/scheduler.py", line 495, in configure
        self._load_pool_from_point()
      File "/net/home/h02/tpilling/metomi/cylc-flow/cylc/flow/scheduler.py", line 782, in _load_pool_from_point
        self.pool.load_from_point()
      File "/net/home/h02/tpilling/metomi/cylc-flow/cylc/flow/task_pool.py", line 193, in load_from_point
        self.compute_runahead()
      File "/net/home/h02/tpilling/metomi/cylc-flow/cylc/flow/task_pool.py", line 321, in compute_runahead
        base_point = min(
    ValueError: min() arg is an empty sequence
CRITICAL - Workflow shutting down - min() arg is an empty sequence

Reproducible Example

#!/bin/bash
# This is not a vip error - this appears to be soemthing which
# happens anyway.

TMP=$(mktemp -d)
echo Files extracted to ${TMP}
mkdir "${TMP}/."

cat > "${TMP}/./flow.cylc" <<__ICI__
[scheduler]
    allow implicit tasks = True
    cycle point format = %Y

[scheduling]
    initial cycle point = 2020
    [[graph]]
        R1 = foo
__ICI__

cylc install "${TMP}" \
    --workflow-name throwaway/ten.marrow

cylc play throwaway/ten.marrow \
    --no-detach \
    --startcp 2022 \

[oliver-sanders]

The issue appears to be the lack of cycling sequences.

The example can be reduced to:

[scheduler]    
    allow implicit tasks = True    
                                                                                                                                   
[scheduling]    
    initial cycle point = 2020    
    [[graph]]    
        R1 = foo   

The issue goes away if you change that to:

    [[graph]]    
        P1 = foo   

[oliver-sanders]

Actually this is an error case anyway, there are no tasks beyond the startcp (because the only task in the workflow is at R1 i.e. the ICP).