[buteo-sync-plugin-carddav] Fix handling of empty contact name. JB#48429

Fix some cases that occur when both first and last name are empty for
a contact:

- The "N" field was set to a string containing semicolons, and this
string was escaped when encoded by QVersitWriter. To avoid this
escaping, add the property as a string list CompoundType instead of a
string.

- Don't include the first/last name in the "N" values as the display
label is already added to the "FN" field anyway.

src/carddav.cpp

@@ -370,9 +370,10 @@ void CardDavVCardConverter::contactProcessed(const QContact &c, QVersitDocument
                 // then just assume that the display label is a useful first name.
                 name.setFirstName(displaylabel);
             }
-            QString nvalue = QStringLiteral("%1;%2;;;").arg(name.lastName(), name.firstName());
+            static const QStringList nvalue = { "", "", "", "", "" };
             QVersitProperty nProp;
             nProp.setName("N");
+            nProp.setValueType(QVersitProperty::CompoundType);
             nProp.setValue(nvalue);
             d->addProperty(nProp);
         }