Fix jer optional values

[davxy]

This fails otherwise

schema.asn

TestModule DEFINITIONS ::= BEGIN

U8 ::= INTEGER (0..255)
U32 ::= INTEGER (0..4294967295)

Testcase ::= SEQUENCE {
    epoch-mark [0] U32 OPTIONAL,
    tickets-mark [1] U8 OPTIONAL
}

END

data.json

{
    "epoch-mark": null,
    "tickets-mark": null
}

validate.py

schema = asn1tools.compile_files("schema.asn", codec="jer")
json_bytes = open("data.json", "rb").read()
dec = schema.decode("Testcase", json_bytes)
enc = schema.encode("Testcase", dec)

---

⚠️ I'm not familiar with the codebase, and my solution is primarily an "it works for me" approach ⚠️

• solves the failing test case I've pasted above
• don't know if there is a better one
• don't know if it breaks something else :-)