fix(ext/web): handle empty TextDecoder input when stream is true (#31691)

for: 
#31605 (comment)

fixes the issue where TextDecoder with stream: true and an empty chunk
would return incorrect results.
It resolves the chunking problem for legacy encodings like big5,
shift_jis, and euc-kr.

Author: mertcanaltin

ext/web/08_text_encoding.js

@@ -119,6 +119,10 @@ class TextDecoder {
       stream = options.stream;
     }
 
+    if (stream && input.length === 0) {
+      return "";
+    }
+
     try {
       /** @type {ArrayBufferLike} */
       let buffer = input;

tests/unit/text_encoding_test.ts

@@ -324,3 +324,56 @@ Deno.test(
     assertStrictEquals(cancelled, true);
   },
 );
+
+Deno.test(
+  "TextDecoder should handle empty chunk in stream mode (legacy encodings)",
+  () => {
+    // big5: [0xa4, 0xa4] => "中" (U+4E2D)
+    {
+      const u8 = new Uint8Array([0xa4, 0xa4]);
+      const str = new TextDecoder("big5").decode(u8);
+      assertEquals(str, "\u4e2d");
+
+      const d = new TextDecoder("big5");
+      const chunks = [
+        d.decode(u8.subarray(0, 1), { stream: true }),
+        d.decode(u8.subarray(1), { stream: true }),
+        d.decode(new Uint8Array(), { stream: true }),
+        d.decode(),
+      ];
+      assertEquals(chunks.join(""), str);
+    }
+
+    // shift_jis: [0x82, 0xa0] => "あ" (U+3042)
+    {
+      const u8 = new Uint8Array([0x82, 0xa0]);
+      const str = new TextDecoder("shift_jis").decode(u8);
+      assertEquals(str, "\u3042");
+
+      const d = new TextDecoder("shift_jis");
+      const chunks = [
+        d.decode(u8.subarray(0, 1), { stream: true }),
+        d.decode(u8.subarray(1), { stream: true }),
+        d.decode(new Uint8Array(), { stream: true }),
+        d.decode(),
+      ];
+      assertEquals(chunks.join(""), str);
+    }
+
+    // euc-kr: [0xb0, 0xa1] => "가" (U+AC00)
+    {
+      const u8 = new Uint8Array([0xb0, 0xa1]);
+      const str = new TextDecoder("euc-kr").decode(u8);
+      assertEquals(str, "\uac00");
+
+      const d = new TextDecoder("euc-kr");
+      const chunks = [
+        d.decode(u8.subarray(0, 1), { stream: true }),
+        d.decode(u8.subarray(1), { stream: true }),
+        d.decode(new Uint8Array(), { stream: true }),
+        d.decode(),
+      ];
+      assertEquals(chunks.join(""), str);
+    }
+  },
+);