Sqlite: Fix null handling

Most of it is only silencing a warning but one is fixing a faulty test. So
writing null values should now be working.

Change-Id: If702e1fc8176a9e0ae82a5589c719d567929202b
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/sqlite/sqlitevalue.h

@@ -102,6 +102,8 @@ public:
             return QVariant(toFloat());
         case ValueType::String:
             return QVariant(QString(toStringView()));
+        case ValueType::Null:
+            break;
         }
 
         return {};
@@ -281,6 +283,8 @@ public:
             return first.toFloat() == second.toFloat();
         case ValueType::String:
             return first.toStringView() == second.toStringView();
+        case ValueType::Null:
+            return false;
         }
 
         return false;