ExpressionTransformer #1917: use the LHS type if the RHS is null

Because otherwise the Java bottom type is stored in java.lang.Object, which is not
of the bottom type, so x = y = null will fail because null is stored in java.lang.Object
and can't be assigned to either y or x

src/com/redhat/ceylon/compiler/java/codegen/ExpressionTransformer.java

@@ -4526,6 +4526,8 @@ private JCExpression transformAssignment(Node op, Tree.Term leftTerm, Tree.Term
             return transformAssignment(op, leftTerm, rhs);
         } else {
             ProducedType valueType = rightTerm.getTypeModel();
+            if(isNull(valueType))
+                valueType = leftTerm.getTypeModel();
             return transformAssignAndReturnOperation(op, leftTerm, boxing == BoxingStrategy.BOXED, 
                     leftTerm.getTypeModel(), valueType, new AssignAndReturnOperationFactory(){
                 @Override