QDBusSignature: fix default ctor to not allocate memory

The QDBusSignature default ctor is noexcept since
49f7281d36028055e61175c23bc455b4147480b9 (Qt 5.5-ish), so detach()
(which must needs allocate memory) cannot be used without breaking the
contract.

Fix by assigning ""_L1 instead, the canonical way to access the shared
empty state. While fromLatin1() isn't noexcept, fromLatin1(""_L1)
never throws.

Amends ed6d1fa71a79a70b7e6a20fbbc737ed9f6c287b1.

Pick-to: 6.7 6.5
Task-number: QTBUG-124919
Change-Id: I5f0c3bdc199998deaa22971a60388f8f3ed0eaa8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 11dbf821942dbda6e53d7bfcf378904318634196)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/dbus/qdbusextratypes.h

@@ -78,9 +78,8 @@ class Q_DBUS_EXPORT QDBusSignature
     QString m_signature;
 public:
     QDBusSignature() noexcept
-    {
-        m_signature.detach(); // mark non-null (empty signatures are valid)
-    }
+        : m_signature(QLatin1StringView("")) // mark non-null (empty signatures are valid)
+    {}
     // compiler-generated copy/move constructor/assignment operators are ok!
     // compiler-generated destructor is ok!