Added explicit NoneType return for mypy in span.duration

[Yun-Kim]

Description

From #2180, the mypy check to span.py found a missing return in the case that a duration was not set yet to a span. Although trivial, it is good practice to have explicit returns. A test case was added to ensure the span.duration property is None by default.

Checklist

• Added to the correct milestone.
• Tests provided or description of manual testing performed is included in the code or PR.
• Library documentation is updated.
• Corp site documentation is updated (link to the PR).