Fix behaviour for brep files without metadata

[johnnonweiler]

When attempting to process a brep file which did not have an associated metadata file, there was an obscure error:

RuntimeError: [json.exception.type_error.302] type must be string, but is null

With this fix, there is now a warning that no metadata has been provided, and then PPP continues.

Checklist

• [x] Source is rebased on latest upstream main so is mergeable automatically or easily merged even with confliction.
• [x] Title of this PR is meaningful: e.g. "fix issue #8 geomPipeline.py -o does not work", not "fix geomPipeline.py"
• [x] Commit message is meaningful: git push origin +your_branch to squash some tiny fixes for CI failures
• [x] Give the maintainer the legal rights to change the open source license in the future (see wiki/Contribution.md) for details

[qingfengxia]

Andy has comment: Concurrent with this it would be good to have a flag that should allow or disallow continuation if no metadata is found

[makeclean]

I think it should fail is metadata is expected and no metatdata is found (maybe default) and then add a flag like --no-fatal or something like that?

[qingfengxia]

I think the current impl (With this fix, there is now a warning that no metadata has been provided, and then PPP continues.), give user the chance to eval imprint brep, for the first time, there is no metadata.

In the future , it can change to Andy's