fix: default host when empty

[marandaneto]

💡 Motivation and Context

PostHogConfig already defaults Host when omitted, but explicitly setting it to null or an empty value caused validation to fail. This keeps host configuration optional by falling back to the default PostHog host for blank values.

💚 How did you test it?

• DOTNET_ROLL_FORWARD=Major dotnet test tests/PostHog.Unity.Tests/PostHog.Unity.Tests.csproj --no-restore

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the release label to the PR

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
tests/PostHog.Unity.Tests/PostHogConfigTests.cs:67-84
**Prefer a parameterised theory + add whitespace case**

The team rule prefers `[Theory]` over repeating near-identical `[Fact]` tests. The two new facts share identical structure and could be a single theory. Additionally, `Validate()` uses `IsNullOrWhiteSpace`, so a whitespace-only host is also defaulted, but there is no test covering that case (while the analogous `WithWhitespaceApiKey_ThrowsArgumentException` test exists for `ApiKey`).

_{Reviews (1): Last reviewed commit: "fix: default host when empty" | Re-trigger Greptile}