fix(core): Do not call onModuleInit if is null

[BrunnerLivio]

Fixes #1252

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?

[x] 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?

See issue

Issue Number: #1252

What is the new behavior?

It does not call onModuleInit, onApplicationBootstrap, onApplicationShutdown or onModuleDestroy if it is of type null.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 4155

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.236%

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Totals
Change from base Build 4098:	0.0%
Covered Lines:	3638
Relevant Lines:	3820

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💛 - Coveralls

[lock]

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