[processing] Don't skip null parameter values when converting

parameters to pythong strings

We need to include these, in order to differentiate unspecified
parameters from parameters set to null values

(cherry-picked from c738bcf)

Author: nyalldawson

src/core/processing/qgsprocessingalgorithm.cpp

@@ -258,7 +258,7 @@ QString QgsProcessingAlgorithm::asPythonCommand( const QVariantMap &parameters,
     if ( def->flags() & QgsProcessingParameterDefinition::FlagHidden )
       continue;
 
-    if ( !parameters.contains( def->name() ) || !parameters.value( def->name() ).isValid() )
+    if ( !parameters.contains( def->name() ) )
       continue;
 
     parts << QStringLiteral( "'%1':%2" ).arg( def->name(), def->valueAsPythonString( parameters.value( def->name() ), context ) );

tests/src/analysis/testqgsprocessing.cpp

@@ -290,8 +290,7 @@ class DummyAlgorithm : public QgsProcessingAlgorithm
       params.insert( "p1", "a" );
       QCOMPARE( asPythonCommand( params, context ), QStringLiteral( "processing.run(\"test\", {'p1':'a'})" ) );
       params.insert( "p2", QVariant() );
-      // not set, should be no change
-      QCOMPARE( asPythonCommand( params, context ), QStringLiteral( "processing.run(\"test\", {'p1':'a'})" ) );
+      QCOMPARE( asPythonCommand( params, context ), QStringLiteral( "processing.run(\"test\", {'p1':'a','p2':None})" ) );
       params.insert( "p2", "b" );
       QCOMPARE( asPythonCommand( params, context ), QStringLiteral( "processing.run(\"test\", {'p1':'a','p2':'b'})" ) );