AK: Assert non-empty Utf32View, when initialized with non-zero length

This was useful in debugging a nullptr dereference, which was happening
through later, but was caused by this inconsistent initialization.

AK/Utf32View.h

@@ -39,6 +39,7 @@ public:
         : m_codepoints(codepoints)
         , m_length(length)
     {
+        ASSERT(codepoints || length == 0);
     }
 
     const u32* codepoints() const { return m_codepoints; }