Writers to accept `Path | None`

[nh13]

In many of our tools, we have optional metric files that are written when an input path is given. This leads to having to have boilerplate for a nullcontext

from contextlib import nullcontext
with MetricWriter[SomeMetric](out_path, SomeMetric) if out_path is not None else nullcontext() as out_writer:
    ...

or having to condition on if the writer is built:

out_writer = (
    None
    if out_path is None
    else (MetricWriter[SomeMetric](filename=out_path, metric_class=SomeMetric))
)

...
if out_writer is not None:
    out_writer.write(...)

Can we instead have the writer accept out_path as either Path or None, such that it can return a new MetricWriter or nullcontext?

[clintval]

What if folks start using os.devnull as the path for when no path is requested?

[nh13]

Is it clearer on the command line to have None or os.devnull? What does defopt think?