fix(Collection): toJSON() errors if the collection includes empty values

[vaporoxx]

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[haroongames-git]

To be honest, I don't think this should actually be added.
I'm using the github version of d.js, and whenever I initialize a Collection, my code throws SyntaxError: Unexpected token '.'

This could break a whole lot of users' code.

[anandre]

v13 introduces breaking changes, including a new Node version requirement, just like the update from v11 to v12 did (Node 8 -> Node 12). v13 will require Node 14 or higher, to take advantage of new features for cleaner code and better functions. Since a recent-ish commit, master requires Node 14 or higher.