Add empty mask handling within pcv.visualize.obj_size_ecdf

[Nirkan]

Describe your changes
An empty mask input to pcv.visualize.obj_size_ecdf returns an error. This change returns and empty chart if the input mask is empty.

Type of update
Is this a:

• Bug fix

Associated issues
Avoid fatal errors and instead just pass an empty mask as return #1742

Additional context
Add any other context about the problem here.

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 004a6a0..e624472. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

---

💡 If you’re a repository administrator, you can configure the quality gates from the settings.