[BUG] - make version dsl values nullable

[oknozor]

Describe the bug
bumping from a repository with no tag, using the version dsl {{latest}} produce the following error:

Error: No previous tag found to replace {{latest}} version

Expected behavior
Their should be a fallback value in the dsl.
example:

post_bump_hooks = [
"""
echo "{{latest| 0.0.0}} to {{version}}
"""
]