Don't force use of SQL dialect when running ogr queries (fix #14407)

Using "SQL" dialect is not recommended as it forces use of the sometimes
buggy SDK query engines. This was breaking the uniqueValues method for
ESRI gdb files.

See https://trac.osgeo.org/gdal/ticket/6415 for GDAL dev recommendation
to use default OGR dialect in place of "SQL" dialect.

(cherry picked from commit 8d9443b)

src/providers/ogr/qgsogrprovider.cpp

@@ -1356,7 +1356,7 @@ bool QgsOgrProvider::createSpatialIndex()
   {
     QByteArray sql = "CREATE SPATIAL INDEX ON " + quotedIdentifier( layerName );  // quote the layer name so spaces are handled
     QgsDebugMsg( QString( "SQL: %1" ).arg( FROM8( sql ) ) );
-    OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), OGR_L_GetSpatialFilter( ogrOrigLayer ), "" );
+    OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), OGR_L_GetSpatialFilter( ogrOrigLayer ), nullptr );
   }
 
   QFileInfo fi( mFilePath );     // to get the base name
@@ -1369,9 +1369,9 @@ bool QgsOgrProvider::createAttributeIndex( int field )
 {
   QByteArray quotedLayerName = quotedIdentifier( OGR_FD_GetName( OGR_L_GetLayerDefn( ogrOrigLayer ) ) );
   QByteArray dropSql = "DROP INDEX ON " + quotedLayerName;
-  OGR_DS_ExecuteSQL( ogrDataSource, dropSql.constData(), OGR_L_GetSpatialFilter( ogrOrigLayer ), "SQL" );
+  OGR_DS_ExecuteSQL( ogrDataSource, dropSql.constData(), OGR_L_GetSpatialFilter( ogrOrigLayer ), nullptr );
   QByteArray createSql = "CREATE INDEX ON " + quotedLayerName + " USING " + mEncoding->fromUnicode( fields()[field].name() );
-  OGR_DS_ExecuteSQL( ogrDataSource, createSql.constData(), OGR_L_GetSpatialFilter( ogrOrigLayer ), "SQL" );
+  OGR_DS_ExecuteSQL( ogrDataSource, createSql.constData(), OGR_L_GetSpatialFilter( ogrOrigLayer ), nullptr );
 
   QFileInfo fi( mFilePath );     // to get the base name
   //find out, if the .idm file is there
@@ -2434,7 +2434,7 @@ void QgsOgrProvider::uniqueValues( int index, QList<QVariant> &uniqueValues, int
   sql += " ORDER BY " + mEncoding->fromUnicode( fld.name() ) + " ASC";  // quoting of fieldname produces a syntax error
 
   QgsDebugMsg( QString( "SQL: %1" ).arg( mEncoding->toUnicode( sql ) ) );
-  OGRLayerH l = OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), nullptr, "SQL" );
+  OGRLayerH l = OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), nullptr, nullptr );
   if ( !l )
   {
     QgsDebugMsg( "Failed to execute SQL" );
@@ -2472,7 +2472,7 @@ QVariant QgsOgrProvider::minimumValue( int index )
     sql += " WHERE " + mEncoding->fromUnicode( mSubsetString );
   }
 
-  OGRLayerH l = OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), nullptr, "SQL" );
+  OGRLayerH l = OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), nullptr, nullptr );
   if ( !l )
   {
     QgsDebugMsg( QString( "Failed to execute SQL: %1" ).arg( mEncoding->toUnicode( sql ) ) );
@@ -2511,7 +2511,7 @@ QVariant QgsOgrProvider::maximumValue( int index )
     sql += " WHERE " + mEncoding->fromUnicode( mSubsetString );
   }
 
-  OGRLayerH l = OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), nullptr, "SQL" );
+  OGRLayerH l = OGR_DS_ExecuteSQL( ogrDataSource, sql.constData(), nullptr, nullptr );
   if ( !l )
   {
     QgsDebugMsg( QString( "Failed to execute SQL: %1" ).arg( mEncoding->toUnicode( sql ) ) );