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/pulls.ts

@@ -25,7 +25,7 @@ export const CHECK_STATUS = "mesh/check";
 
 /** The statuses a branch's rule requires, from the rule the forge applies to it: none when it is unprotected or checks
  *  no status; undefined when the rule could not be read. */
-export function requiredOf(rule: { enable_status_check?: boolean; status_check_contexts?: string[] } | null | undefined): string[] | undefined {
+export function requiredOf(rule: { enable_status_check?: boolean; status_check_contexts?: string[] | null } | null | undefined): string[] | undefined {
   if (rule === undefined) return undefined;
   if (!rule || !rule.enable_status_check) return [];
   return (rule.status_check_contexts ?? []).filter(Boolean);