Replace remaining 0 with nullptr in Qt code

Also used type-appropriate enum values such as Qt::NoItemFlags in
some cases.

All cases identified via -Wzero-as-null-pointer-constant

src/qt/peertablemodel.cpp

@@ -96,7 +96,7 @@ public:
         if (idx >= 0 && idx < cachedNodeStats.size())
             return &cachedNodeStats[idx];
 
-        return 0;
+        return nullptr;
     }
 };
 
@@ -104,7 +104,7 @@ PeerTableModel::PeerTableModel(interfaces::Node& node, ClientModel *parent) :
     QAbstractTableModel(parent),
     m_node(node),
     clientModel(parent),
-    timer(0)
+    timer(nullptr)
 {
     columns << tr("NodeId") << tr("Node/Service") << tr("Ping") << tr("Sent") << tr("Received") << tr("User Agent");
     priv.reset(new PeerTablePriv());
@@ -197,8 +197,7 @@ QVariant PeerTableModel::headerData(int section, Qt::Orientation orientation, in
 
 Qt::ItemFlags PeerTableModel::flags(const QModelIndex &index) const
 {
-    if(!index.isValid())
-        return 0;
+    if (!index.isValid()) return Qt::NoItemFlags;
 
     Qt::ItemFlags retval = Qt::ItemIsSelectable | Qt::ItemIsEnabled;
     return retval;