[OGR provider] Use OGR_F_IsFieldSetAndNotNull() when available.

Starting with GDAL 2.2, there are 2 concepts: unset fields and null fields
whereas previously there was only unset fields. For QGIS purposes, both
states (unset/null) are equivalent.

Cherry-picked from 47dd83d

Author: rouault

src/providers/ogr/qgsogrfeatureiterator.cpp

@@ -33,6 +33,12 @@
 // - mAttributeFields
 // - mEncoding
 
+// Starting with GDAL 2.2, there are 2 concepts: unset fields and null fields
+// whereas previously there was only unset fields. For QGIS purposes, both
+// states (unset/null) are equivalent.
+#ifndef OGRNullMarker
+#define OGR_F_IsFieldSetAndNotNull OGR_F_IsFieldSet
+#endif
 
 QgsOgrFeatureIterator::QgsOgrFeatureIterator( QgsOgrFeatureSource* source, bool ownSource, const QgsFeatureRequest& request )
     : QgsAbstractFeatureIteratorFromSource<QgsOgrFeatureSource>( source, ownSource, request )
@@ -335,7 +341,7 @@ void QgsOgrFeatureIterator::getFeatureAttribute( OGRFeatureH ogrFet, QgsFeature
 
   QVariant value;
 
-  if ( OGR_F_IsFieldSet( ogrFet, attindex ) )
+  if ( OGR_F_IsFieldSetAndNotNull( ogrFet, attindex ) )
   {
     switch ( mSource->mFields.at( attindex ).type() )
     {

src/providers/ogr/qgsogrprovider.cpp

@@ -55,6 +55,13 @@ email                : sherman at mrcc.com
 #include <sys/vfs.h>
 #endif
 
+// Starting with GDAL 2.2, there are 2 concepts: unset fields and null fields
+// whereas previously there was only unset fields. For QGIS purposes, both
+// states (unset/null) are equivalent.
+#ifndef OGRNullMarker
+#define OGR_F_IsFieldSetAndNotNull OGR_F_IsFieldSet
+#endif
+
 static const QString TEXT_PROVIDER_KEY = "ogr";
 static const QString TEXT_PROVIDER_DESCRIPTION =
   QString( "OGR data provider" )
@@ -2595,7 +2602,7 @@ void QgsOgrProvider::uniqueValues( int index, QList<QVariant> &uniqueValues, int
   OGRFeatureH f;
   while (( f = OGR_L_GetNextFeature( l ) ) )
   {
-    uniqueValues << ( OGR_F_IsFieldSet( f, 0 ) ? convertValue( fld.type(), mEncoding->toUnicode( OGR_F_GetFieldAsString( f, 0 ) ) ) : QVariant( fld.type() ) );
+    uniqueValues << ( OGR_F_IsFieldSetAndNotNull( f, 0 ) ? convertValue( fld.type(), mEncoding->toUnicode( OGR_F_GetFieldAsString( f, 0 ) ) ) : QVariant( fld.type() ) );
     OGR_F_Destroy( f );
 
     if ( limit >= 0 && uniqueValues.size() >= limit )
@@ -2637,7 +2644,7 @@ QVariant QgsOgrProvider::minimumValue( int index )
     return QVariant();
   }
 
-  QVariant value = OGR_F_IsFieldSet( f, 0 ) ? convertValue( fld.type(), mEncoding->toUnicode( OGR_F_GetFieldAsString( f, 0 ) ) ) : QVariant( fld.type() );
+  QVariant value = OGR_F_IsFieldSetAndNotNull( f, 0 ) ? convertValue( fld.type(), mEncoding->toUnicode( OGR_F_GetFieldAsString( f, 0 ) ) ) : QVariant( fld.type() );
   OGR_F_Destroy( f );
 
   OGR_DS_ReleaseResultSet( ogrDataSource, l );
@@ -2676,7 +2683,7 @@ QVariant QgsOgrProvider::maximumValue( int index )
     return QVariant();
   }
 
-  QVariant value = OGR_F_IsFieldSet( f, 0 ) ? convertValue( fld.type(), mEncoding->toUnicode( OGR_F_GetFieldAsString( f, 0 ) ) ) : QVariant( fld.type() );
+  QVariant value = OGR_F_IsFieldSetAndNotNull( f, 0 ) ? convertValue( fld.type(), mEncoding->toUnicode( OGR_F_GetFieldAsString( f, 0 ) ) ) : QVariant( fld.type() );
   OGR_F_Destroy( f );
 
   OGR_DS_ReleaseResultSet( ogrDataSource, l );