fix: skip undefined values in merge

[aadamcik]

🔗 Linked issue

#1066

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)

📚 Description

Resolves #1066 .

PS: It seems like a lot of changes in the PR but I just moved the condition to top:

if (!isObject(values)) {
  return this
}

so it's more readable.

And added:

if (value === undefined) {
  return
}

📝 Checklist

• I have linked an issue or discussion.