In OPC UA, some datatypes support NULL values, and some not. It is hard to figure out, which data types do support NULL, and which not.
For example, DECIMAL is defined not to support NULLs in the UA Binary mapping (5.2.3), but there is no statement for XML (5.3.2) or JSON (5.4.3). It would be desirable to have such a statement in the generic part (e.g. 5.1.7).
A String is nullable as mentioned in the sections about Variants (5.1.6).
Would be great to have an overview table in 5.1.x to expose the information, which datatype is nullable and which not (that should be independent of the encoding). The encodings should define, how nulls are handled.