gh-71810: fix corner case (length==0) for int.to_bytes()

```pycon
>>> (0).to_bytes(0, 'big', signed=True)
b''
>>> (-1).to_bytes(0, 'big', signed=True)  # was b''
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    (-1).to_bytes(0, 'big', signed=True)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
OverflowError: int too big to convert
```

Author: skirpichev

Lib/test/test_long.py

@@ -1385,6 +1385,8 @@ def equivalent_python(n, length, byteorder, signed=False):
         self.assertEqual((-1).to_bytes(5, 'big', signed=True),
                          b'\xff\xff\xff\xff\xff')
         self.assertRaises(OverflowError, (1).to_bytes, 0, 'big')
+        self.assertRaises(OverflowError, (-1).to_bytes, 0, 'big', signed=True)
+        self.assertRaises(OverflowError, (-1).to_bytes, 0, 'little', signed=True)
 
         # gh-98783
         class SubStr(str):

Objects/longobject.c

@@ -1209,7 +1209,7 @@ _PyLong_AsByteArray(PyLongObject* v,
         *p = (unsigned char)(accum & 0xff);
         p += pincr;
     }
-    else if (j == n && n > 0 && is_signed) {
+    else if (j == n && is_signed) {
         /* The main loop filled the byte array exactly, so the code
            just above didn't get to ensure there's a sign bit, and the
            loop below wouldn't add one either.  Make sure a sign bit