Handle empty yaml files

[innerhippy]

Environment

• Python version: 3.9
• schema-enforcer version: 1.1.2

Expected Behavior

run schema-enforcer validate without errors

Observed Behavior

Stack trace when loading empty yaml files

self._top_level_properties = set(content.keys())
AttributeError: 'NoneType' object has no attribute 'keys' 

Steps to Reproduce

1. Add empty yaml file in directory structure
2. run validate command

Addition of simple conditional in schema_enforcer/instances/file.py line 100 fixes issue

if content:
    self._top_level_properties = set(content.keys())