TracesSamplerCallback allows returning null in Java SDK

[adinauer]

Description

Docs do not mention it's possible to return null from TracesSamplerCallback (https://develop.sentry.dev/sdk/telemetry/traces/#tracessampler).

The question is whether we should align with other SDKs in not allowing to return null or if we consider this a feature worth keeping.

PRO for keeping it:

• Easy for customers to customize sampling decision only for things they care about and let the SDK perform its default handling on the rest

Consequences of removing this possibility:

• Customers always have to return a sampling rate
• They also need to handle copying the parent sampling decision
• Customers have to build their own mechanism for supplying a sample rate via ENV vars or config if they need it or read it from SentryOptions

Also see python: https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/tracing.py#L1070C9-L1090

[adinauer]

Let's keep current behaviour and document what's possible in https://docs.sentry.io/platforms/java/configuration/sampling/