fix(component): do not exclude falsy types from LetDirective's input type

[markostanimirovic]

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• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has 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
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

If we pass the property that contains falsy types to the LetDirective as follows:

@Component({
  selector: 'app-foo',
  template: `
    <div *ngrxLet="prop as alias">
      {{ prop }}
      {{ alias }}
    </div>
  `,
})
export class FooComponent {
  prop: Observable<string> | null | false = of('marko');
}

The type of prop within the embedded template will be Observable<string> (falsy types are excluded).
Fortunately, the type of the alias variable is correctly inferred (string | null | false).

What is the new behavior?

The type of both alias and prop is correctly inferred:

• prop: Observable<string> | null | false
• alias: string | null | false

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[markostanimirovic]

This line was probably copied from NgIfDirective. It removes falsy types from the directive's input, which is not expected in the case of LetDirective.

[timdeschryver]

Could/want we a test for this?

[markostanimirovic]

It would require the "strictTemplates" setup for tests. AFAIK, that's not possible ("angularCompilerOptions" in tsconfig.spec.json doesn't affect tests).