fillIn/deepFillIn behaviour on nullish values

At the moment, the fillIn function behavior is different from the mixIn and merge when treating nullish values.

fillIn({
    a: null,
    b: 'foo'
}, {
    a: 'foo',
    b: 'bar'
});

// result -> { a: 'foo', b: 'foo' }
// expected -> { a: null, b: 'foo' }

In my opinion, a null/undefined value should be treated like a normal value, just like mixIn and merge does.
What you guys think?

If you all have the same opinion I can do a PR with the necessary changes and tests.

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