Client support for writes of size 0

[JBenda]

When working with HDF5 I realized that HDF5 allows for writes with length of 0, this results current in an j_object_write call of size 0 which will trigger an error because the data buffer is NULL because there are no data to transfer.

Now the question:

• should the user/our HDF5 integration catch writes of size 0, or
• should the object client just swallow a write request if data == NULL && size == 0

FYI: if we currently write something of length 0 but with a "valid" pointer, we will send a message with payload 0