QgsVectorFilewriter skip default null options

Don't specify dataset options which have null default values. Setting
these as empty strings might introduce a different behavior than
expected.

For example not creating an xsd companion file for GML.

src/core/qgsvectorfilewriter.cpp

@@ -1032,7 +1032,7 @@ QMap<QString, QgsVectorFileWriter::MetaData> QgsVectorFileWriter::initMetaData()
                                         "Note that the schema file isn't actually accessed by OGR, so it "
                                         "is up to the user to ensure it will match the schema of the OGR "
                                         "produced GML data file." ),
-                           QLatin1String( "" )  // Default value
+                           QString()  // Default value
                          ) );
 
   datasetOptions.insert( QStringLiteral( "XSISCHEMA" ), new SetOption(
@@ -3225,7 +3225,7 @@ QStringList QgsVectorFileWriter::concatenateOptions( const QMap<QString, QgsVect
       case QgsVectorFileWriter::String:
       {
         QgsVectorFileWriter::StringOption *opt = dynamic_cast<QgsVectorFileWriter::StringOption *>( option );
-        if ( opt )
+        if ( opt && !opt->defaultValue.isNull() )
         {
           list.append( QStringLiteral( "%1=%2" ).arg( it.key(), opt->defaultValue ) );
         }