fix(stepper): content not being rendered out initially with ivy

[crisbeto]

Fixes the stepper's content not being shown on the first render with Ivy, because we assume that the template with be present on init.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
status "branch-manager" is failing
status "ci/circleci: tests_browserstack" is pending
status "ci/circleci: tests_local_browsers" is pending
status "ci/circleci: tests_saucelabs" is pending

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