Add empty mask handling within threshold.triangle

[Nirkan]

Describe your changes
An empty mask will fail if provided to pcv.threshold.triangle so this bugfix detects empty mask and returns it without error.

Type of update
Is this a:

• Bug fix

Associated issues
This is associate with issue #1742

Additional context
Follows #1811 .

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 5e908b4..4821fd4. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	❌ Failure	❗ 1 occurence introduced	View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

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