yaml validation errors are not helpful if the file is empty

[PietroPasotti]

Bug Description

validation error is not helpful on an empty file.
(it was actually hard to find out that the file was empty because I thought I'd deleted it). The project already had a (valid) charmcraft.yaml next to it, and couldn't figure out why charmcraft wasn't liking it.

To Reproduce

charmcraft pack

Environment

charmcraft 3.2.2

metadata.yaml

# Copyright 2025 Canonical Ltd.
# See LICENSE file for licensing details.

Relevant log output

❯ charmcraft pack                                                                                                                                                                                                                                                                                                                          
Invalid file: 'metadata.yaml'                                                                                                                                                                                                                                                                                                                  
Recommended resolution: Ensure metadata.yaml meets the juju metadata.yaml specification.                                                                                                                                                                                                                                                       
For more information, check out: https://juju.is/docs/sdk/metadata-yaml                                                                                                                                                                                                                                                                        
Full execution log: '/home/pietro/.local/state/charmcraft/log/charmcraft-20250110-134846.839621.log'