Relax overly-strict assert in Serializer constructor (RIPD-1701):

The constructor would previously assert that the specified buffer pointer
was non-null, even if the buffer size is specified as 0. While reasonable,
this also makes it more difficult to use this API.

src/ripple/protocol/Serializer.h

@@ -51,12 +51,13 @@ public:
 
     Serializer (void const* data, std::size_t size)
     {
-        assert(!data == !size);
-
         mData.resize(size);
 
         if (size)
+        {
+            assert(data != nullptr);
             std::memcpy(mData.data(), data, size);
+        }
     }
 
     Slice slice() const noexcept