changed check for validity from isGeosValid() to validateGeometry()

[ghost]

changed check for validity from isGeosValid() to validateGeometry() in dissolve.py.

Second try, as the first one failed due to wrong usage of git.
The background is that complicated polygons (e.g. created from vectorize, like innerringtouchesouterring.gml ) may contain "invalid" geometries according to GEOS , mainly because of self-intersections of the outer ring. QgsGeometry::checkValidity is more robust and only sorts out the truly defective geometries. Attached are tests with invalid and valid geometries. I hope this time I'm doing better in committing :)
The gml output files having defective (NULL) CRS is another bug somewhere else, #14544.

[m-kuhn]

There seems to be an indentation problem here:

Compare this test:

  - name: dissolve
  algorithm: qgis:dissolve
  #input is valid as validateGeometry(), but invalid in isGeosValid()
  params:

to the previous (working one)

  # case 3: merge with longest common boundary
  - algorithm: qgis:eliminatesliverpolygons
    name: Eliminate sliver polygons largest area

[m-kuhn]

the file is currently not located in the subfolder custom

[m-kuhn]

• The nullgeometrydissolve exptected output file seems to be missing
• Could you move the dataset into custom and revert the path?
• There are some other indentation "problems" which get rejected by autopep8: https://travis-ci.org/qgis/QGIS/jobs/117682954#L1063-L1112 if you plan to provide more fixes, please read the bottom of the selected lines to setup your local git to auto-fix such things in the future. If not, I can do this quickly for you when it's being merged.

[m-kuhn]

Thank you very much, such pull requests are valuable!