[cookies] fix Cookies.get return type (#34860)

* [cookies] fix Cookies.get return type

If not exists cookie, return undefined.

* fix tests

types/cookies/cookies-tests.ts

@@ -10,9 +10,9 @@ const server = http.createServer((req, res) => {
     new Cookies(req, res, {keys: new Keygrip([])});
     new Cookies(req, res, {secure: true});
 
-    let unsigned: string;
-    let signed: string;
-    let tampered: string;
+    let unsigned: string | undefined;
+    let signed: string | undefined;
+    let tampered: string | undefined;
 
     if (req.url === "/set") {
         cookies

types/cookies/index.d.ts

@@ -22,7 +22,7 @@ interface Cookies {
      * Cookie header in the request. If such a cookie exists,
      * its value is returned. Otherwise, nothing is returned.
      */
-    get(name: string, opts?: Cookies.GetOption): string;
+    get(name: string, opts?: Cookies.GetOption): string | undefined;
 
     /**
      * This sets the given cookie in the response and returns

types/cookies/tsconfig.json

@@ -6,7 +6,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [
@@ -20,4 +20,4 @@
         "index.d.ts",
         "cookies-tests.ts"
     ]
-}
+}