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/trunk.ts

@@ -128,7 +128,7 @@ export interface TrunkForge {
   branchHead(owner: string, repo: string, branch: string): Promise<string>;
   /** The rule the forge applies to a branch: which of the controller's statuses are kept there (issue 1301). Not given,
    *  or not read: both, as before. */
-  appliedProtection?(owner: string, repo: string, branch: string): Promise<{ enable_status_check?: boolean; status_check_contexts?: string[] } | null>;
+  appliedProtection?(owner: string, repo: string, branch: string): Promise<{ enable_status_check?: boolean; status_check_contexts?: string[] | null } | null>;
 }
 
 /** What it writes with: the merge check's one writer (gate.ts), which decides in the head's own turn. */