fix: return empty content array for null/undefined tool results

[serhiizghama]

Problem

When a tool callback returns null or undefined (void/side-effectful tools), _wrapInToolResult was producing a TextBlock with the placeholder text '<null>' or '<undefined>'. These strings:

• look like internal placeholders to both humans and models
• are not valid JSON (unlike Python's json.dumps(None) → "null")
• create confusion for side-effectful tools (delete, send, log) that have no meaningful return value

Fixes strands-agents/harness-sdk#2529.

Solution

Replace the separate null and undefined handlers with a single combined check that returns an empty content array:

if (value === null || value === undefined) {
  return new ToolResultBlock({
    toolUseId,
    status: 'success',
    content: [],
  })
}

An empty content array correctly signals success without a misleading placeholder. This matches the common expectation for side-effectful tools.

Testing

• Updated existing null and undefined test cases to assert content: []
• Added a new void callback test covering the real-world side-effectful tool pattern (see issue example with greet)
• All 2834 unit tests pass; no type errors

[strands-agent]

This repository has been merged into the strands-agents/harness-sdk monorepo and will be archived shortly. All new development happens there.

If this PR is still relevant, please recreate it against the monorepo. The code now lives under strands-ts/. Full commit history was preserved, so your base should be findable.

Apologies for the disruption, and thank you for contributing!