refactor: refactors parts of the ADK codebase to improve null safety and consistency

[copybara-service]

refactor: refactors parts of the ADK codebase to improve null safety and consistency

This CL refactors parts of the ADK codebase to improve null safety and consistency. The main changes include:

1. BaseAgent:

   • beforeAgentCallback and afterAgentCallback fields and their accessors now use ImmutableList (defaulting to empty) instead of Optional<List>.
   • findAgent and findSubAgent now return Optional<BaseAgent>, with findSubAgent being reimplemented using Java Streams.

2. BaseAgentConfig: Getters for subAgents, beforeAgentCallbacks, and afterAgentCallbacks now return an empty list if the underlying field is null.

3. CallbackUtil: getBeforeAgentCallbacks and getAfterAgentCallbacks return ImmutableList.of() instead of null for null inputs.

4. LlmAgent: The codeExecutor() method now returns Optional<BaseCodeExecutor>.

These changes necessitate updates in BaseLlmFlow, CodeExecution, and Runner to handle the new Optional return types.