docs(agent-extension-api): mark ConfigProperties @Nullable where null is possible

[zeitlinger]

Summary

• Annotate config parameters and locals as @Nullable on the SPIs where a null ConfigProperties already flows through in declarative-config mode.
• Document the declarative-config contract in the javadoc of AgentExtension.extend and BootstrapPackagesConfigurer.configure.

Context

In declarative-config mode, AutoConfigureUtil.getConfig(autoConfiguredSdk) returns null. That null already flows through AgentInstaller into:

• BootstrapPackagesConfigurer.configure(builder, config)
• IgnoredTypesConfigurer.configure(builder, config) (deprecated overload; AgentInstaller already substitutes EmptyConfigProperties.INSTANCE before calling it)
• AgentExtension.extend(builder, config)

The signatures did not reflect that. This PR aligns the public contract with actual runtime behavior.

Split off from #17719 so the NullAway enablement PR does not need a runtime workaround (empty-map fallback or requireNonNull) that would silently break extensions using config == null to detect declarative mode.

No behavior change.

Test plan

• CI green
• No runtime behavior change — only annotations + javadoc

[laurit]

Supplying null to a method that previously was never invoked with null is essentially an incompatible change. Perhaps it would be better to invoke these methods with empty properties instead?

[zeitlinger]

Good point — I took the safer route here. Instead of widening the non-deprecated extension SPIs to nullable, I now normalize the declarative-config case to an empty ConfigProperties before calling them.

[Copilot]

Pull request overview

This PR attempts to align javaagent extension-facing configuration nullability with declarative-config behavior, but the implementation currently normalizes nullable config to an empty config before some SPIs receive it.

Changes:

• Marks the local SDK config in AgentInstaller as nullable.
• Introduces an EmptyConfigProperties fallback before passing config to agent setup hooks.
• Annotates the deprecated IgnoredTypesConfigurer.configure(..., config) parameter as @Nullable.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/AgentInstaller.java	Updates SDK config handling and passes a normalized config into bootstrap, ignored-types, and extension configuration paths.
javaagent-extension-api/src/main/java/io/opentelemetry/javaagent/extension/ignore/IgnoredTypesConfigurer.java	Marks the deprecated config parameter as nullable.

Comments suppressed due to low confidence (1)

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/AgentInstaller.java:180

• This changes declarative-config mode from passing null to AgentExtension.extend to passing EmptyConfigProperties. That removes the config == null signal for custom extensions, and the built-in instrumentation loader will also forward the non-null empty config to instrumentation modules, contradicting the no-runtime-behavior-change/declarative-mode contract described for this PR.

        agentBuilder = agentExtension.extend(agentBuilder, sdkConfigOrEmpty);