Add emtpy mask handling within threshold.otsu

[Nirkan]

Describe your changes
An empty mask np.zeros((100, 100)) without specifying dtype=np.uint8 fails and causes error. The bugfix returns the copy of original mask if its an empty mask.

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
This is one part of the issues in #1742 and the fix is similar to #1811

For the reviewer
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• 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 cf15e46..715ac87. 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 ↗

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