fix(common): preserve zero optional durations

[sidsri14]

What changed

Updated msOptionalToTs to treat only null and undefined as absent values, so a numeric 0 duration is preserved and converted to a zero protobuf timestamp.

Added a unit test covering msOptionalToTs(0).

Fixes #2233.

Validation

• npx --yes pnpm@10.27.0 --filter @temporalio/proto build
• npx --yes pnpm@10.27.0 --filter @temporalio/common build
• npx --yes pnpm@10.27.0 --filter @temporalio/common test
• npx --yes pnpm@10.27.0 exec eslint packages/common/src/time.ts packages/common/src/__tests__/test-time.ts
• npx --yes pnpm@10.27.0 prettier --check packages/common/src/time.ts packages/common/src/__tests__/test-time.ts

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