[APIC-296] MAINT Python client's file_to_civis should not warn about file size of 0 bytes

[gkcivis]

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [ ] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed

This change adds logic to the civis.io.file_to_civis function that warns the user if they are using the function on a file with a size of zero bytes.