feat(effects): allow multiple empty EffectModule.forRoot()

[alex-okrushko]

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PR Type

What kind of change does this PR introduce?

Allow multiple empty EffectModule.forRoot().
Within Google we have a number of cases with empty EffectModule.forRoot() have to be added for tests to be able to find this module - ngc has a hard time generating a summary for EffectsModule.forRoot otherwise.

Up until now, the solution was to just remove the guard altogether when merging NgRx locally, but that's not a sustainable approach and requires "un-doing" it with every merge.

[ ] 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:

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[timdeschryver]

LGTM