fix: Fix empty pr edge case

[jbrocher]

=== auto-pr-body ===

Summary
Introduced new features and improved existing code in order to provide better support for parsing of pull request bodies.

List of Changes

• Introduced new property body, so that it returns the custom text and auto text separated by a delimiter, including the case when the PR body is empty.
• Updated init() method to split the body string into chunks and to assign the auto text value based on the length of the chunks.
• Introduced a new method, update_auto_body(), to update the auto text.
• Added test cases for both body and update_auto_body() methods.

Refactoring Target

• Replace substring extraction from the body string in the update_auto_body() method with the body_chunks list.
• Use regular expression instead of split() function to better parse the body string.
• Rename the DELIMITER constant for better readability and maintainability.