Exception when a modifier with undefined values is used

[xolott]

When this modifier is used, redis-oplog throws an exception:

const modifer = {
    $addToSet: undefined
};

Collection.update(selector, modifier);

Exception:

Exception in setInterval callback: TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at packages/cultofcoders:redis-oplog/lib/utils/getFields.js:12:20
    at Array.forEach (<anonymous>)
    at getFields (packages/cultofcoders:redis-oplog/lib/utils/getFields.js:10:29)
    at ns.Collection.update (packages/cultofcoders:redis-oplog/lib/mongo/Mutator.js:198:32)
    at ns.Collection.update (packages/cultofcoders:redis-oplog/lib/mongo/extendMongoCollection.js:40:35)

I think the issue is in this line:

redis-oplog/lib/utils/getFields.js

Line 11 in 58ba426

if (op[0] == '$') {

Something like this should work.

    const isNil = (value) => value === null || value === undefined;
    Object.entries(mutator).forEach(function ([op, params]) {
        if (op[0] == '$' && !isNil(params)) {

A modifier with a value undefined is valid for the mongo driver. It just ignores it.

[github-actions]

Thank you for submitting this issue!

We, the Members of Meteor Community Packages take every issue seriously.
Our goal is to provide long-term lifecycles for packages and keep up
with the newest changes in Meteor and the overall NodeJs/JavaScript ecosystem.

However, we contribute to these packages mostly in our free time.
Therefore, we can't guarantee you issues to be solved within certain time.

If you think this issue is trivial to solve, don't hesitate to submit
a pull request, too! We will accompany you in the process with reviews and hints
on how to get development set up.

Please also consider sponsoring the maintainers of the package.
If you don't know who is currently maintaining this package, just leave a comment
and we'll let you know

[xolott]

I created the PR #399 with what I think would fix the issue