fix(cache): invalidate wrongly cached handler entities (#1857)

Author: pi0

src/runtime/cache.ts

@@ -257,6 +257,13 @@ export function defineCachedEventHandler<
       if (entry.value.body === undefined) {
         return false;
       }
+      // https://github.com/unjs/nitro/pull/1857
+      if (
+        entry.value.headers.etag === "undefined" ||
+        entry.value.headers["last-modified"] === "undefined"
+      ) {
+        return false;
+      }
       return true;
     },
     group: opts.group || "nitro/handlers",