Constraints: unique constraint not correctly evaluated when 0

[elpaso]

Using the same dataset from #36167 with a slightly different join configuration (see the second image) inserting a new feature "ogc_fid" 0 is not recognized as a constraint violation even if a feature with "ogc_fid" 0 is already present in the database table, the expected behavior is to see the orange cross with the unique constraint violation tooltip.

[elpaso]

Digging further, this appears a serious bug to me.
It apparently comes from the wrong initialization of the QgsFeature constructor

#ifndef SIP_RUN
    QgsFeature( const QgsFields &fields, QgsFeatureId id = QgsFeatureId() );
#else
    QgsFeature( const QgsFields &fields, qint64 id = 0 );
#endif

assuming that inside QGIS we consider fid values as:

• < 0: FID_IS_NEW new features that are not yet committed to the data provider
• >= 0 features that exist in the data provider
  this means that 0 is not FID_NULL but a valid fid value for a not new feature, but the constructor of QgsFeature defaults feature id to 0 which leads to this bug.

I believe it should construct new features with a default of FID_NULL.