fix(Subscription): Return Empty when teardown === null

[ChristianKohler]

Description:
Subscription.add breaks if teardown function is null. (works with undefined)

typeof teardown returns object when null. It breaks later when rxjs tries to access the closed property.

The commit fixes that by first checking for null/undefined.

Related issue (if exists):
Haven't found an issue

[coveralls]

Pull Request Test Coverage Report for Build 8105

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.2%) to 96.962%

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Files with Coverage Reduction	New Missed Lines	%
src/internal/Subscription.ts	1	93.38%
src/internal/observable/fromPromise.ts	1	87.5%
src/internal/observable/fromIterable.ts	2	95.24%
src/internal/observable/fromObservable.ts	2	83.33%

Totals
Change from base Build 8098:	0.2%
Covered Lines:	5777
Relevant Lines:	5958

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💛 - Coveralls