Protection and fixes for trim with boundary and openings

[IsakNaslundBh]

Issues addressed by this PR

Closes #100
Closes #99

• Add protection against empty lists returned from BooleanDifference
• Make it so that PlanarSurfaces returned always has external boundaries of type BoundaryCurve (alignment across both cases when trimmed with boundary and not)
• Update UTs

Test files

https://burohappold.sharepoint.com/:f:/s/BHoM/EkRElFZ1zRRPlDy90IR_DQEBv7Nr_12yZ6dN9PFOWTWnLw?e=7ZPHt5

Changelog

Additional comments

[IsakNaslundBh]

@BHoMBot check unit-tests

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[FraserGreenroyd]

Code changes make sense and unit test is happy.

[FraserGreenroyd]

@BHoMBot check required

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[FraserGreenroyd]

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[bhombot-ci]

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