Fix default layer sometimes inserted into multi layer parameter values

Author: nyalldawson

src/core/processing/qgsprocessingparameters.cpp

@@ -622,7 +622,7 @@ QList<QgsMapLayer *> QgsProcessingParameters::parameterAsLayerList( const QgsPro
   else
     resultStringList << val.toString();
 
-  if ( ( resultStringList.isEmpty() || resultStringList.at( 0 ).isEmpty() ) )
+  if ( layers.isEmpty() && ( resultStringList.isEmpty() || resultStringList.at( 0 ).isEmpty() ) )
   {
     resultStringList.clear();
     // check default

tests/src/core/testqgsprocessing.cpp

@@ -2310,6 +2310,9 @@ void TestQgsProcessing::parameterLayerList()
   params.insert( "optional",  QVariant() );
   QCOMPARE( QgsProcessingParameters::parameterAsLayerList( def.get(), params, context ), QList< QgsMapLayer *>() << v1 );
 
+  params.insert( "optional",  QVariantList() << QVariant::fromValue( r1 ) );
+  QCOMPARE( QgsProcessingParameters::parameterAsLayerList( def.get(), params, context ), QList< QgsMapLayer *>() << r1 );
+
   code = def->asScriptCode();
   QCOMPARE( code, QStringLiteral( "##optional=optional multiple vector " ) + v1->id() );
   fromCode.reset( dynamic_cast< QgsProcessingParameterMultipleLayers * >( QgsProcessingParameters::parameterFromScriptCode( code ) ) );