Fix importers misbehaving when given a file instead of a directory

[IRDonch]

Summary

CocoImporter and MotsImporter crash with an exception when given a path
to a file (a non-JSON file in the case of CocoImporter). Make them return
an empty result instead.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT