[Replay] Prevent frontend errors when SDK sends null data

[c298lee]

There is nothing preventing SDKs from sending bad data to replays. On the frontend, we expect breadcrumbs to have some required fields depending on their type, but there is no validation on the SDK side or on ingest. To prevent errors on the frontend, we should set null fields that are required to an empty string during breadcrumb/frame hydration.

Related to https://github.com/getsentry/sentry/issues/78837 and https://github.com/getsentry/sentry/pull/78841