[processing] Avoid non-spatial vector layers in DXF Export alg

[gacarrillor]

DXF export functionality doesn't accept non-spatial layers. However, this is the scenario nowadays:

• DXF Export app dialog does not even show non-spatial layers, which is good!
• DXF processing algorithm:
  • Shows non-spatial layers and allows users to configure them as valid inputs.
  • Fails at exporting non-spatial layers throwing a "DXF export failed, the extent could not be determined" error message.

To improve the DXF export algorithm behavior, this PR makes non-spatial layers non-acceptable input values for the DXF Export alg. As a result, users won't even have those layers listed in the algorithm dialog, which is cleaner, and in the case of big projects, highly desirable.

Some tests were added as well.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit f9390ff)

[github-actions]

Tests failed for Qt 5

One or more tests failed using the build from commit f9390ff

WMS_GetMap_Highlight_Empty_Labels

WMS_GetMap_Highlight_Empty_Labels

Test failed at test_wms_getmap_highlight_empty_labels at tests/src/python/test_qgsserver_wms_getmap.py:1391

Rendered image did not match tests/testdata/control_images/qgis_server/WMS_GetMap_Highlight_Empty_Labels/WMS_GetMap_Highlight_Empty_Labels.png (found 271 pixels different)

The full test report (included comparison of rendered vs expected images) can be found here.

Further documentation on the QGIS test infrastructure can be found in the Developer's Guide.