Fix merge lines in dissolve

src/analysis/processing/qgsalgorithmdissolve.cpp

@@ -215,6 +215,8 @@ QVariantMap QgsDissolveAlgorithm::processAlgorithm( const QVariantMap &parameter
   return processCollection( parameters, context, feedback, [ & ]( const QVector< QgsGeometry > &parts )->QgsGeometry
   {
     QgsGeometry result( QgsGeometry::unaryUnion( parts ) );
+    if ( QgsWkbTypes::geometryType( result.wkbType() ) == QgsWkbTypes::LineGeometry )
+      result = result.mergeLines();
     // Geos may fail in some cases, let's try a slower but safer approach
     // See: https://issues.qgis.org/issues/20591 - Dissolve tool failing to produce outputs
     if ( ! result.lastError().isEmpty() && parts.count() >  2 )
@@ -227,7 +229,7 @@ QVariantMap QgsDissolveAlgorithm::processAlgorithm( const QVariantMap &parameter
       for ( const auto &p : parts )
       {
         result = QgsGeometry::unaryUnion( QVector< QgsGeometry >() << result << p );
-        if ( QgsWkbTypes::flatType( result.wkbType() ) == QgsWkbTypes::LineString )
+        if ( QgsWkbTypes::geometryType( result.wkbType() ) == QgsWkbTypes::LineGeometry )
           result = result.mergeLines();
         if ( feedback->isCanceled() )
           return result;