fix(push): fix return typing for observables to include undefined

[samuelfernandez]

ngrxPush typing considers undefined when the input type is an observable

Closes #2888

PR Checklist

Please check if your PR fulfills the following requirements:

• 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?

Closes #2888

What is the new behavior?

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Applications that have strict null checks might need to handle undefined.

Other information

[samuelfernandez]

Existing unit tests already account for this scenario and assert that undefined is returned. The only problem was with the typing, which has been fixed.

[ngrxbot]

Preview docs changes for 0ed5d65 at https://previews.ngrx.io/pr2907-0ed5d65e/