You can pass a null as an input argument to a table function or return a null in a result row of a table function only when the data type of the corresponding parameter to its Java method is object-mapped (see Data Type Mapping Between SQL and Java).
If the data type of the corresponding parameter to the Java method of the table function is defined as simple-mapped, then attempts to evaluate a null at runtime fail and return an error to the requestor. This is because simple-mapped data types cannot represent nulls.
See Null Call Clause for general information about the Null Call clause. Note that the only form of null call supported by table functions is CALLED ON NULL INPUT.