fix(core): when the discoverable decorator was not used on calling getProviders/getControllers

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: closes #12518

What is the new behavior?

returns an empty Set - a null object, kinda (although I dislike the ideia of initializing a new empty object every time)

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 054f6b66-c7e5-4b28-8e5c-22ef6ec99c67

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.639%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/discovery/discoverable-meta-host-collection.ts	0	2	0.0%

Totals
Change from base Build 79434c05-5367-4a7a-a04c-0f91cc807577:	0.0%
Covered Lines:	6469
Relevant Lines:	6983

---

💛 - Coveralls

[kamilmysliwiec]

LGTM