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

[crisbeto]

Equivalent of #15485 for the Ivy branch.

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 "ci/circleci: api_golden_checks" is failing
     status "ci/circleci: build_release_packages" is failing
     status "ci/circleci: e2e_tests" is failing
     status "ci/circleci: lint" is failing
     status "ci/circleci: prerender_build" is failing
     status "ci/circleci: tests_browserstack" is failing
     status "ci/circleci: tests_local_browsers" is failing
     status "ci/circleci: tests_saucelabs" is failing
     missing required labels: target: *
     status "ci/circleci: bazel_build_test" is pending

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