gitea: take a rule's null contexts as the forge's type says, so the strict build the checker runs compiles (issue 1891)

The checker builds a package with a build script by its tsconfig (strict),
where requiredOf and TrunkForge's appliedProtection said
status_check_contexts is string[] and BranchProtection says string[] | null:
four TS2345 errors, hidden on main behind earlier failures. Types only; the
emitted code is the same, and a test holds that null contexts require nothing.

modules/gitea/test/pulls.test.ts

@@ -216,3 +216,12 @@ test("a base's required statuses decide which of the controller's statuses are w
   const comment = commentFor(c, {}, ["mesh/repo-check"]);
   assert.ok(comment === null || !comment.includes("mesh/merge-gate"), comment ?? "");
 });
+
+test("a rule the forge gives with no contexts (null) requires nothing, and an unread rule is unknown", async () => {
+  const { requiredOf } = await import("../pulls.ts");
+  // The forge's BranchProtection says status_check_contexts may be null (protection.ts); requiredOf takes it as it comes.
+  assert.deepEqual(requiredOf({ enable_status_check: true, status_check_contexts: null }), []);
+  assert.deepEqual(requiredOf({ enable_status_check: true, status_check_contexts: ["mesh/check", ""] }), ["mesh/check"]);
+  assert.deepEqual(requiredOf(null), []);
+  assert.equal(requiredOf(undefined), undefined);
+});