fix(core): avoid NPE when Agent description is missing

[svetanis]

Prevents a NullPointerException in the google-genai library when an Agent is defined without a description. Includes a regression test.

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Problem:
Calling AgentTool.declaration() triggers a NullPointerException when an Agent is defined without an explicit description. The crash occurs because the underlying google-genai library's generated FunctionDeclaration builder uses Optional.of() for the description field, which is not null-safe.

Solution:
Coerce a null description to an empty string in AgentTool.declaration() using Guava's Strings.nullToEmpty(), so the google-genai builder always receives a non-null value. This matches the Python, Go, and Kotlin ports, which send an empty description when an agent has none.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of passed java test results:
Passed AgentToolTest.declaration_withNullDescription_usesEmptyDescription, which verifies that a null description no longer triggers an NPE and results in an empty description.

Manual End-to-End (E2E) Tests:

Verified by running a sample agentic workflow where the Agent class lacked a description field. Previously, this resulted in a crash during tool registration; it now initializes and executes correctly.

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
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Additional context

Stack Trace:
java.lang.NullPointerException
at java.base/java.util.Objects.requireNonNull(Objects.java:220)
at java.base/java.util.Optional.of(Optional.java:113)
at com.google.genai.types.AutoValue_FunctionDeclaration$Builder.description(AutoValue_FunctionDeclaration.java:160)
at com.google.adk.tools.AgentTool.declaration(AgentTool.java:135)

Note on Root Cause:
While this PR ensures stability within adk-java, the underlying issue appears to be in the google-genai library. The generated AutoValue_FunctionDeclaration.Builder uses Optional.of() for the description field, which enforces a non-null constraint. A more robust upstream fix would involve the generator using Optional.ofNullable(). This local fix is necessary to prevent crashes in the meantime.

[hemasekhar-p]

Hi @svetanis, thank you for your contribution! We appreciate you taking the time to submit this pull request. Currently this PR is under review by our team, we will keep you posted if any additional information is required. thank you.

[svetanis]

@hemasekhar-p ,

Thanks for the update!
Just a quick heads up - I went ahead and rebased the branch onto the latest main.

[kvmilos]

Thanks @svetanis for catching the bug and for the PR. Just a few small things below.

[svetanis]

Thanks for the review, @kvmilos! All three points addressed:

1. Dependency — dropped commons-lang3.StringUtils.isNotBlank for Guava's Strings.nullToEmpty.

2. Port parity — now sends an empty description when the agent has none, matching the Python, Go, and Kotlin ports.
   This also simplified the fix to a single line with no name fallback:

   String desc = Strings.nullToEmpty(this.description());

3. Test — fixed the no-op assertion (assertThat(declaration).isPresent();), renamed the test to declaration_withNullDescription_usesEmptyDescription, and asserted the empty description via hasValue("").

All 27 AgentToolTest tests pass.
The branch has also been rebased against the latest main.