Support assignment from empty braces

This makes resetting a handle more natural and idiomatic, matching
modern C++ expectations without affecting existing semantics.

Change-Id: I85e3c2e8b0d4c288d90b1739b83994d345a757ed
Reviewed-by: Artem Dyomin <[email protected]>

Author: padubsky

src/corelib/tools/quniquehandle_p.h

@@ -193,7 +193,11 @@ class QUniqueHandle : private QtPrivate::CompactStorage<Deleter>
     static_assert(noexcept(std::declval<Type>() >= std::declval<Type>()));
 
     template <if_default_constructible<Deleter> = true>
-    explicit QUniqueHandle(const Type& handle = HandleTraits::invalidValue()) noexcept
+    QUniqueHandle() noexcept
+    {}
+
+    template <if_default_constructible<Deleter> = true>
+    explicit QUniqueHandle(const Type &handle) noexcept
         : m_handle{ handle }
     {}