test(dtype): cover unicode fields in the structured default scalar test (#4496)

#4495 also changed the default of `U` fields in structured dtypes from
"0" to "". Add `U4` to the regression test and name string fields in
the changelog entry.

Assisted-by: ClaudeCode:claude-opus-5-5

Author: d-v-b

changes/4495.bugfix.md

@@ -1 +1 @@
-Structured data types now default byte-string (`S`) and raw-bytes (`V`) fields to zero bytes. Before, an array with a raw-bytes field failed to create without an explicit fill value, and byte-string fields defaulted to `b"0"`.
+Structured data types now default string (`S`, `U`) and raw-bytes (`V`) fields to zero bytes. Before, an array with a raw-bytes field failed to create without an explicit fill value, and string fields defaulted to `"0"`.

tests/test_dtype/test_npy/test_structured.py

@@ -394,10 +394,11 @@ def test_subarray_structured_dtype_raises() -> None:
         Struct.from_native_dtype(dtype)
 
 
-@pytest.mark.parametrize("field_dtype", ["S4", "V4"])
+@pytest.mark.parametrize("field_dtype", ["S4", "U4", "V4"])
 def test_default_scalar_bytes_field_is_zero(field_dtype: str) -> None:
     """
-    The default scalar of a bytes field is zero bytes, as for the same dtype outside a struct.
+    The default scalar of a string or bytes field is zero bytes, as for the same dtype outside
+    a struct.
     """
     dtype = np.dtype([("a", field_dtype), ("b", "<f4")])
     assert Struct.from_native_dtype(dtype).default_scalar().tobytes() == bytes(dtype.itemsize)