[processing] correctly handle layers without attributes in GDAL-based geoprocessing algorithms (fix #17287)

[alexbruy]

Description

Correctly handle layers without without attributes in GDAL-based geoprocessing algorithms. Fixes https://issues.qgis.org/issues/17287. Supersedes #5397

Checklist

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[nyalldawson]

Looks good to me - @gioman are you ok for this to be merged?

[gioman]

Looks good to me - @gioman are you ok for this to be merged?

sure!