wip: no quick transform, when there is no vertices

[guziy]

Rationale

The tricontourf fails with the dataset I have, because the vertices parameter in quick_vertices_transform is empty.

Implications

Checklist

• [ ] If you have not already done so, ensure you've read and signed the Contributor Licence Agreement (CLA). (See the governance page for the CLA and what to do with it).

• [ ] Ensure there is a suitable item in the cartopy test suite for the change you are proposing.

  I am not able to run the tests.

      ImportError while importing test module 
      '../Python/cartopy/lib/cartopy/tests/mpl/test_web_services.py'.
      Hint: make sure your test modules/packages have valid Python names.
     Traceback:
       __init__.py:95: in <module>
      import cartopy.crs
      crs.py:36: in <module>
     from cartopy._crs import (CRS, Geodetic, Globe, PROJ4_VERSION,
   E   ModuleNotFoundError: No module named 'cartopy._crs'
  

[QuLogic]

What is the error that this is fixing?

You will need an editable install of Cartopy in order to run tests.

[guziy]

What is the error that this is fixing?

It fixes the IndexError and allows to plot my data. I need to make a test yet for this.

You will need an editable install of Cartopy in order to run tests.

At some point, I was able to run the tests, but now I have to remember how I did it.

[rcomer]

These changes look very similar to part of #2213. @guziy are you able to reproduce the problem with Cartopy v0.22? I appreciate that some time has passed, so it’s fair enough if you don’t have the means to check any more!

[guziy]

Sorry @rcomer, I can't find the test code... I should have posted it....((

I did rebased and pushed, not sure if you have a test for this problem though...