Don't raise error if metaflow_extension's Distribution#files is None.

[trhodeos]

With the metaflow-card-html package, we're seeing the following error:

.../metaflow/extension_support/__init__.py", line 414, in _get_extension_packages
    for f in dist.files:
TypeError: 'NoneType' object is not iterable

Turns out Distribution.files can return None. metaflow-card-html doesn't include the appropriate wheel metadata files, and, when included, hits this error.

[romain-intel]

The issue though is that this won't load the files and they won't be included when moving to remote execution so I need to find another way; ie: fallback on iterating through the package the way I do for non-distribution packages.

And of course, the package could be fixed too :)

[trhodeos]

The issue though is that this won't load the files and they won't be included when moving to remote execution so I need to find another way; ie: fallback on iterating through the package the way I do for non-distribution packages.

And of course, the package could be fixed too :)

Yeah, that makes sense. For now, I've added an _ext_debug, with the [] so that metaflow doesn't crash and I've added a TODO.

I'll open an upstream patch for the extension so this metadata gets added as well!