fix: previousChanges should check instance is new record or not while specific attributes' values were undefined

[JimmyDaddy]

// title: Untitled, authorId: undefined
const post = new Post({ title: 'Untitled' });
assert.deepEqual(post.previousChanges('title'), {});
assert.deepEqual(post.previousChanges('authorId'), {});
post.title = 'MHW';
// authorId was undefined
post.authorId = 100;
await post.save();
// post is not a new record now
assert.deepEqual(post.previousChanges('authorId'), { authorId: [ null, 100 ] });

[codecov-commenter]

Codecov Report

Merging #276 (6c48617) into master (8d98cc0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files          44       44           
  Lines        3204     3204           
=======================================
  Hits         3123     3123           
  Misses         81       81           

Impacted Files	Coverage Δ
src/bone.js	97.98% <100.00%> (ø)

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[cyjake]

It looks like this.isNewRecord covers this.#rawPrevious[name] === undefined already, can we give it a try?

btw, this line is a bit too much now, I believe a multiple line if block is necessary once the column width exceeds some threshold like 90 or 120