Change sample_rand to non-nullable values

[philprime]

Description

Our current implementation uses nullable values for sentry_rand in the SentryBaggage.
This allows cases where the values are null and not set in the baggage.

There is an ongoing clarification effort happening in getsentry/sentry-docs#12592 to clarify that these values must be set, therefore we should change the types and refactor the code base to adhere to compiler errors.

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1. docs: Clarify sample_rand specs sentry-docs#12592
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