GML: Integer fields with NULL values are reported as String

[mkuhn]

If a gml file has an integer field which contains a NULL value, this field will no longer be reported as an integer field, but as a string field instead.

As an example dataset, the file ​polys_centroid.gml can be taken.

The file was created with an older version of GDAL, so NULL fields are still missing from the file itself. For this file, the field intval is correctly reported as integer.

The same file created with newer versions of GDAL (tested 2.2.1) will contain an additional entry <ogr:intval xsi:nil="true"/> for the feature polys.2 (See ​a manually patched version here). As soon as this entry is there, the field is reported as String type.

[Even Rouault]

In 39984:

GML: fix field type detection logic to avoid a field with xsi:nil=true to be identified as a string (fixes #7027)

[Even Rouault]

In 39985:

GML: fix field type detection logic to avoid a field with xsi:nil=true to be identified as a string (fixes #7027)