fix: make Response.body optional (#63)

Author: eduardoboucas

src/function/response.d.ts

@@ -6,6 +6,6 @@ export interface Response {
   multiValueHeaders?: {
     [header: string]: ReadonlyArray<boolean | number | string>
   }
-  body: string
+  body?: string
   isBase64Encoded?: boolean
 }