Add a check in the intro/exit steps that player is defined before rendering contents

[sentry-io]

We have this in the game stages, should also have it for intro/exit.

Sentry Issue: DELIBERATION-EMPIRICA-83

React ErrorBoundary TypeError: Cannot read properties of undefined (reading 'get')
  at SubmitButton (../../src/elements/SubmitButton.jsx:10:32)
  at Element (../../src/elements/Element.jsx:21:27)
  at ConditionsConditionalRender (../../src/components/ConditionalRender.jsx:125:47)
  at RecursiveConditionalRender (../../src/components/ConditionalRender.jsx:134:39)
  at ConditionsConditionalRender (../../src/components/ConditionalRender.jsx:125:47)
...
(15 additional frame(s) were not displayed)

TypeError: Cannot read properties of undefined (reading 'get')
  at SubmitButton (../../src/elements/SubmitButton.jsx:13:32)
...
(5 additional frame(s) were not displayed)