fix(scope): `set_user` should not overwrite existing user data

[sentrivana]

According to https://develop.sentry.dev/sdk/unified-api/#scope set_user should "shallow merge" user data. We're instead overwriting it.

Changes in this PR:

• set_user called with a non-empty dict will now update, rather than replace, current user data on the scope
• To reset the current user, set_user needs to be called with None or {} as argument.

These are both breaking changes. Added them to the MIGRATION_GUIDE.

Session behavior is left unchanged.

Fixes https://github.com/getsentry/sentry-python/issues/933

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