fix(deer-go): prevent server initialization panic when APMPLUS_APP_KEY is not set

[Zerofisher]

What type of PR is this?

fix: A bug fix

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Fixes #109

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Pull Request Overview

This PR fixes a server initialization panic that occurs when the APMPLUS_APP_KEY environment variable is not set. The fix prevents the server from attempting to use tracing middleware when the tracer configuration is incomplete.

• Modified server initialization logic to conditionally apply tracing middleware only when both tracer and config are available
• Added fallback server initialization without tracing when configuration is missing

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