Handle edge cases when summarising labels layers

[adamltyson]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?

• Add external layer does not generate summary.csv file #96
• "summarising" an region that hasn't yet been segmented (i.e. an empty layer) raises an error #98

What does this PR do?

• Closes Add external layer does not generate summary.csv file #96 by converting data to numpy before analysing
• Closes "summarising" an region that hasn't yet been segmented (i.e. an empty layer) raises an error #98 by catching if all layers are empty, and not attempting to summarise them

References

Please reference any existing issues/PRs that relate to this PR.

How has this PR been tested?

Tests added & some manual testing.

Is this a breaking change?

Nope

Does this PR require an update to the documentation?

Nope

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[deprecated-napari-hub-preview-bot]

Preview page for your plugin is ready here:
https://preview.napari-hub.org/brainglobe/brainreg-segment/99
Created: 2023-06-12T16:58:20.557094

[alessandrofelder]

LGTM - two super-minor comments to consider before merging!