Send a clearer error message if response is truncated before a chunk

If a chunked response is truncated exactly before sending a new chunk
(that is, before the first byte of the chunk size), urllib3 throws
`InvalidChunkLength(got length b'\\n', 0 bytes read)`. This is
confusing as no chunk length was sent (a valid chunk length line would
also be \r\n-terminated and we didn't see those either).

If the chunk length is the empty string, send a different error message.

src/urllib3/response.py

@@ -1001,9 +1001,14 @@ def _update_chunk_length(self) -> None:
         try:
             self.chunk_left = int(line, 16)
         except ValueError:
-            # Invalid chunked protocol response, abort.
             self.close()
-            raise InvalidChunkLength(self, line) from None
+            if line:
+                # Invalid chunked protocol response, abort.
+                raise InvalidChunkLength(self, line) from None
+            else:
+                # Truncated at start of next chunk
+                raise ProtocolError("Response ended prematurely") from None
+
 
     def _handle_chunk(self, amt: int | None) -> bytes:
         returned_chunk = None

test/test_response.py

@@ -1042,6 +1042,21 @@ def test_invalid_chunk_length(self) -> None:
         assert isinstance(orig_ex, InvalidChunkLength)
         assert orig_ex.length == fp.BAD_LENGTH_LINE.encode()
 
+    def test_truncated_before_chunk(self) -> None:
+        stream = [b"foooo", b"bbbbaaaaar"]
+        fp = MockChunkedNoChunks(stream)
+        r = httplib.HTTPResponse(MockSock)  # type: ignore[arg-type]
+        r.fp = fp  # type: ignore[assignment]
+        r.chunked = True
+        r.chunk_left = None
+        resp = HTTPResponse(
+            r, preload_content=False, headers={"transfer-encoding": "chunked"}
+        )
+        with pytest.raises(ProtocolError) as ctx:
+            next(resp.read_chunked())
+
+        assert str(ctx.value) == "Response ended prematurely"
+
     def test_chunked_response_without_crlf_on_end(self) -> None:
         stream = [b"foo", b"bar", b"baz"]
         fp = MockChunkedEncodingWithoutCRLFOnEnd(stream)
@@ -1300,6 +1315,11 @@ def _encode_chunk(self, chunk: bytes) -> bytes:
         return f"{len(chunk):X};asd=qwe\r\n{chunk.decode()}\r\n".encode()
 
 
+class MockChunkedNoChunks(MockChunkedEncodingResponse):
+    def _encode_chunk(self, chunk: bytes) -> bytes:
+        return b""
+
+
 class MockSock:
     @classmethod
     def makefile(cls, *args: typing.Any, **kwargs: typing.Any) -> None: