Exception encoding nulled long

[robin-anil]

When encoding a long field set to null (which is typical of a JSON object), instead of the field being ignored the code throws an exception. I don't see a reason not to support the old behavior. Null or undefined override on a nulled field should ignore it from being encoded on the byte stream.

util.longNe = function longNe(val, lo, hi) {
    if (typeof val === "object") // Long-like, null is invalid and throws
        return val.low !== lo || val.high !== hi;
    var bits = util.LongBits.from(val);
    return bits.lo !== lo || bits.hi !== hi;
};

[robin-anil]

This is a general comment about nulled fields in general. not just long.

[dcodeIO]

There is no particularly important reason not to support ´null`. I just skipped it to save the additional assertion, i.e. here

("if(%s!==undefined&&util.longNe(%s,%d,%d))", ref, ref, field.defaultValue.low, field.defaultValue.high);

but I am also OK with supporting this again.

[robin-anil]

Thank you! This is the last remaining issue(so far) in our migration from 5 to 6. Its coming along nicely.

[dcodeIO]

I've added this for optional long fields especially now because any other objects would be messages, which default to null and thus are already checked against null where optional. Everything else should probably throw because it's either required or not an object.