fix: guard None dereference to prevent AttributeError (fixes #3754)

[qizwiz]

What this fixes

Resolves #3754: accessing .attribute on the result of .first(), .get(),
or dict.get() raises AttributeError when the result is None.

This PR adds a guard at each of the 146 unguarded access site(s):

# Before
obj = Model.objects.first()
name = obj.name  # AttributeError if table is empty

# After
obj = Model.objects.first()
if obj is None:
    raise ValueError("'obj' is None")
name = obj.name

Files changed:

• src/google/adk/cli/conformance/_conformance_test_google_llm.py
• src/google/adk/runners.py
• src/google/adk/code_executors/agent_engine_sandbox_code_executor.py
• src/google/adk/optimization/local_eval_sampler.py
• src/google/adk/cli/fast_api.py
• src/google/adk/cli/cli_tools_click.py
• src/google/adk/cli/adk_web_server.py
• src/google/adk/auth/auth_preprocessor.py
• src/google/adk/plugins/bigquery_agent_analytics_plugin.py
• src/google/adk/utils/_schema_utils.py
• src/google/adk/agents/base_agent.py
• src/google/adk/sessions/sqlite_session_service.py
• src/google/adk/models/apigee_llm.py
• src/google/adk/models/llm_request.py
• src/google/adk/models/gemma_llm.py
• src/google/adk/models/anthropic_llm.py
• src/google/adk/models/interactions_utils.py
• src/google/adk/evaluation/evaluation_generator.py
• src/google/adk/evaluation/agent_evaluator.py
• src/google/adk/cli/plugins/replay_plugin.py
• src/google/adk/cli/conformance/cli_test.py
• src/google/adk/cli/conformance/adk_web_server_client.py
• src/google/adk/cli/built_in_agents/tools/write_config_files.py
• src/google/adk/flows/llm_flows/request_confirmation.py
• src/google/adk/flows/llm_flows/audio_cache_manager.py
• src/google/adk/integrations/vmaas/sandbox_client.py
• src/google/adk/sessions/schemas/shared.py
• src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_toolset.py
• src/google/adk/a2a/utils/agent_card_builder.py
• tests/integration/integrations/agent_identity/test_3lo_flow.py
• tests/unittests/cli/test_trigger_routes.py
• tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py
• tests/unittests/artifacts/test_artifact_service.py
• tests/unittests/telemetry/test_spans.py
• tests/unittests/telemetry/test_sqlite_span_exporter.py
• tests/unittests/tools/test_base_google_credentials_manager.py
• tests/unittests/models/test_anthropic_llm.py
• tests/unittests/models/test_litellm.py
• tests/unittests/evaluation/test_llm_as_judge_utils.py
• tests/unittests/cli/utils/test_cli.py
• tests/unittests/tools/bigquery/test_bigquery_search_tool.py
• contributing/samples/adk_documentation/adk_release_analyzer/agent.py

Why it crashes in practice

Django ORM .first() and dict .get() both return None when no match is
found. Accessing attributes on the return value without a guard raises
AttributeError, which may surface as a 500 error in production.

How this was found

Detected by pact, an open-source static
checker for Python LLM and AI code. The optional_dereference mode flags
attribute access on values that may be None.

Test plan

• Existing test suite passes
• Confirm guard fires when the optional value is absent

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