Followup #41109: fix algorithms

Check for NULL instead of valid (because NULL is valid).

The old behavior was relying on the fact that a NULL attribute was
stored by OGR as an empty string instead of a NULL QVariant of the
corresponding field type, the conversion of the QVariant( QString())
to a numeric field was checked (and it was failing) so everything
worked (at least for non-string types).

(cherry picked from commit d66d66e)

src/analysis/processing/qgsalgorithmpointslayerfromtable.cpp

@@ -132,14 +132,14 @@ QVariantMap QgsPointsLayerFromTableAlgorithm::processAlgorithm( const QVariantMa
     double x = attrs.at( xFieldIndex ).toDouble( &xOk );
     double y = attrs.at( yFieldIndex ).toDouble( &yOk );
 
-    if ( xOk && yOk )
+    if ( ! attrs.at( xFieldIndex ).isNull() && ! attrs.at( yFieldIndex ).isNull() && xOk && yOk )
     {
       QgsPoint point( x, y );
 
-      if ( zFieldIndex >= 0 )
+      if ( zFieldIndex >= 0 && ! attrs.at( zFieldIndex ).isNull() )
         point.addZValue( attrs.at( zFieldIndex ).toDouble() );
 
-      if ( mFieldIndex >= 0 )
+      if ( mFieldIndex >= 0 && ! attrs.at( mFieldIndex ).isNull() )
         point.addMValue( attrs.at( mFieldIndex ).toDouble() );
 
       f.setGeometry( QgsGeometry( point.clone() ) );

src/core/qgsdatetimestatisticalsummary.cpp

@@ -65,14 +65,14 @@ void QgsDateTimeStatisticalSummary::addValue( const QVariant &value )
   else if ( value.type() == QVariant::Date )
   {
     QDate date = value.toDate();
-    testDateTime( date.isValid() ? QDateTime( date, QTime( 0, 0, 0 ) )
+    testDateTime( ! date.isNull() ? QDateTime( date, QTime( 0, 0, 0 ) )
                   : QDateTime() );
   }
   else if ( value.type() == QVariant::Time )
   {
     mIsTimes = true;
     QTime time = value.toTime();
-    testDateTime( time.isValid() ? QDateTime( QDate::fromJulianDay( 0 ), time )
+    testDateTime( ! time.isNull() ? QDateTime( QDate::fromJulianDay( 0 ), time )
                   : QDateTime() );
   }
   else //not a date