Use explicit NULL value for parameters of digest algorithm identifier for improved compatibility

Parameters are nominally optional, so they could be skipped
altogether, instead of using NULL. However, a version of some RFC had forgotten to include the specifier 'OPTIONAL'. So, to be safe, most implementation include a DER NULL value (0x05 0x00).

So, to be on the safe side, generators should include the NULL value, and decoders should be able to handle the omitted parameters.