ContentStreamLoader and SolrInputDocument/SolrInputField should not allow null key's or values

Today we can create a SolrInputDocument with null keys or values. We should validate them and throw IllegalArgumentExceptions 

We should also validate ContentStreamLoader so that no null values creep in there .

Today this test would fail because SolrInputDocument is not null and also allows adding null / keys and values

doc = new SolrInputDocument(null , null);
assertNull(d);