test: close the empty-batch hole in the e2e unstubbed-request guard (closes #187)

The guard that reports unrecognised POST bodies used batch.every(), which is
vacuously true on an empty array, so a POST with body [] was answered 200 []
and escaped the one mechanism whose job is to make unrecognised outbound
traffic fail the suite rather than pass silently. Unreachable in practice
today, which is exactly the qualifier that stops being true later.

The comment explaining the guard also described a mechanism that does not
exist: playwright-core decodes a binary body lossily rather than returning
null, so such a body reaches the JSON parse as mojibake and is reported by the
catch, while only an absent or empty body decodes to null and is reported by
the type guard. Both are reported; the comment now describes the two real
routes.

tests/e2e/network.js

@@ -297,17 +297,26 @@ async function handleRpc(route, postData, opts, report) {
     // ethers batches by default, so the body may be an array.
     const batch = Array.isArray(payload) ? payload : [payload];
 
-    // Anything that is not a JSON-RPC object, or a batch of them, is not
-    // RPC at all and must be reported like any other unrecognised
-    // outbound traffic rather than dereferenced. request.postData()
-    // returns null both for a bodyless POST and for a body Playwright
-    // cannot decode as UTF-8 (sendBeacon with a Blob, or any binary
-    // payload), so this is not an empty-string special case: it rejects
-    // every non-object payload, exactly as the catch above rejects every
-    // unparseable one.
+    // Anything that is not a JSON-RPC object, or a NON-EMPTY batch of
+    // them, is not RPC at all and must be reported like any other
+    // unrecognised outbound traffic rather than dereferenced.
+    //
+    // The length check is not decoration: every() is vacuously true on an
+    // empty array, so without it a POST with body [] was answered 200 []
+    // and escaped the guard entirely (issue #187). No real batch is empty,
+    // so nothing legitimate is caught by it.
+    //
+    // Two distinct paths land a non-RPC body here, and neither is an
+    // empty-string special case. playwright-core's postData() is
+    // `buffer.toString("utf-8") || null`, so an absent or empty body
+    // decodes to null, JSON.parse("null") yields null, and the type guard
+    // below reports it. A binary body is instead decoded LOSSILY into
+    // mojibake — not null — which is not valid JSON, so the catch above
+    // reports that one. Both end up reported; only the route differs.
     if (
         payload === null ||
         typeof payload !== "object" ||
+        batch.length === 0 ||
         !batch.every((req) => req !== null && typeof req === "object")
     ) {
         report("unstubbed request: POST " + route.request().url());