Allow multiple prs in pr command

nixprstatus/pr.py

@@ -75,6 +75,8 @@ def get_pr(pr: int) -> dict[str, Any]:
 def pr_merge_status(
     pr: int, branches: list[str] = DEFAULT_BRANCHES, check_backport: bool = True
 ) -> PRStatus:
+    if not branches:
+        branches = DEFAULT_BRANCHES
     url = f"https://api.github.com/repos/NixOS/nixpkgs/pulls/{pr}"
     pr_response = requests.get(url, headers=DEFAULT_HEADERS)
     pr_response.raise_for_status()