XYZ: complementary fix to #5488

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27406 f0d54148-0727-0410-94bb-9a71ac55c965
XYZ: deal with lines that have missing values (but still regularly spaced)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27408 f0d54148-0727-0410-94bb-9a71ac55c965