Config: incorrect warning thrown when no files are found in import

[jenlampton]

A spin off from #661 (comment)

If no files are found during a config import, we should throw a warning specifying that no files are found. Right now we instead throw the incorrect warning:
The uploaded configuration matches the current site configuration exactly. No changes were made.

This implies that files were found, comparisons were made, and it was determined that the imported files match the config exactly. But that's not at all what's actually happening :)

[serundeputy]

is this referring to here:
/admin/config/development/configuration/full -- I get: '' no message, no error.
here:
/admin/config/development/configuration/single -- I get: 'Paste your configuration here field is required.'

is this issue referring to another form?

[quicksketch]

I think @jenlampton meant that she gets this error when uploading a .tar.gz file that doesn't contain any config files. So if you actually upload a file but it doesn't contain any files you'll get the inaccurate message saying the uploaded config is "identical" rather than just nothing found.

[serundeputy]

I took a look at this. It is in this file:
core/modules/config/config.admin.inc

Actually detecting that the uploaded tgz is empty or does not contain a valid config file is a bit over my head.