fix(store): correctly infer action group events defined as empty object

[markostanimirovic]

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?

The action group type won't be correctly inferred when events are defined as an empty object:

const fooActions = createActionGroup({
  source: 'Foo',
  events: {},
});

// there is no compilation error
fooActions.x();
fooActions.y();

What is the new behavior?

The action group type will be correctly inferred when events are defined as an empty object, so in the example above, the compilation error will be thrown.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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