Skip empty outlines and display a message in the console.

[francodgstn]

I needed a quick workaround for the issue #789 (and #708)
It doesn't address the root cause, why some outlines are empty, but at least it allows to export the ImageJ ROI file.

I'm not used to python, you might want to adjust the code.

[francodgstn]

@mrariden I see you have the bug #789 assigned, any chance to review my change?

[carsen-stringer]

did the other PR fix this?

[francodgstn]

Hi, not exactly. The other PR was just to ignore empty line in the imagej_roi_converter.py.
Here the idea is to do not generate those at all and give feedback in the console.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 52.76%. Comparing base (5f2f541) to head (19b65ce).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
cellpose/io.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #829      +/-   ##
==========================================
- Coverage   52.97%   52.76%   -0.22%     
==========================================
  Files          18       18              
  Lines        4119     4126       +7     
==========================================
- Hits         2182     2177       -5     
- Misses       1937     1949      +12     

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