TrySaveXML erases existing metadata

[normanb]

Expected behavior and actual behavior.

When asking for stats on a file I would expect the original image metadata to be maintained.

TileDB::TrySaveXML in #1402 has the same issue so I will try and fix this there. Creating this issue for tracking purposes.

Steps to reproduce the problem.

wget https://download.osgeo.org/geotiff/samples/spot/chicago/UTM2GTIF.TIF
gdal_translate -OF PNG UTM2GTIF.TIF output.png
cp output.png.aux.xml original.png.aux.xml
gdalinfo -stats output.png
diff original.png.aux.xml output.png.aux.xml

I would expect the only difference to be the stats in the new metadata file, the old metadata has been removed.

Operating system

Ubuntu 18.04 64 bit

GDAL version and provenance

Current master

[normanb]

I put a simple fix in https://github.com/OSGeo/gdal/pull/1402/files#diff-9a223edcf74381629981a9396dcdc1db it is sufficient to check whether the serialized return is null or not.

[rouault]

@normanb You can back out your change. I've fixed the issue slightly differently per #1431