feat(store): add clearResult to reset a mock selector

[timdeschryver]

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?

[ ] Bugfix
[x] 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?

Calling mockSelector.setValue(undefined), does a reset the mocked selector.

Closes #2244

What is the new behavior?

Calling mockSelector.setValue(undefined), sets the result to undefined.
To reset a mock selector, use mockSelector.clearResult().

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGE:

BEFORE:

Using `mockSelector.setResult(undefined)` resulted in clearing the
return value.

AFTER:

Using `mockSelector.setResult(undefined)` will set the return value of
the selector to `undefined`.
To reset the mock selector, use `mockSelector.clearResult()`.

[ngrxbot]

Preview docs changes for 84e99c3 at https://previews.ngrx.io/pr2270-84e99c3/

[Maximaximum]

This would be extremely useful, looking forward for this to be merged in!

[alex-okrushko]

This would result in a large number of broken tests.