Add null guard for PrInfo in Checkout-GhAwPr.ps1

Prevents silent fork check bypass when gh returns empty/malformed
JSON — $null.isFork evaluates to $false in PowerShell, which would
let the fork check pass incorrectly.

Note: ready_for_review cannot be added to pull_request_target types
yet — gh-aw compiler doesn't include it in the allowed type list.
Filed as a known gap.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

.github/scripts/Checkout-GhAwPr.ps1

@@ -50,6 +50,11 @@ if ($LASTEXITCODE -ne 0) {
     exit 1
 }
 
+if (-not $PrInfo -or -not $PrInfo.author) {
+    Write-Host "❌ PR #$PrNumber returned empty or malformed metadata"
+    exit 1
+}
+
 if ($PrInfo.isFork) {
     Write-Host "⏭️ PR #$PrNumber is from a fork. workflow_dispatch does not check out fork PRs."
     Write-Host "   Fork PRs are evaluated automatically via pull_request_target."