Plot Profile Ignore zero length lines

[will-moore]

Tiny fix to Plot_Profile script, noticed while testing https://github.com/ome/omero-py/pull/278

To test:

• "accidentally" create a line of zero length in iviewer by dragging to draw (instead of click-move-click)
• Run the Plot_Profile script
• Other lines should be processed as normal

[jburel]

to avoid this type of issue, could you add more tests to https://github.com/ome/omero-scripts/blob/develop/test/integration/test_analysis_scripts.py?

[jburel]

Thanks for adding the tests