fix(datastructures): MutableScopeHeaders.getall() treats an empty-list default as "no default"

`getall(key, default)` used `if default:` to decide whether a default
was supplied, so an explicit `default=[]` (a legitimate way to ask for
"return an empty list if the header is missing" rather than raising)
is falsy and gets treated the same as `default=None` — raising
KeyError instead of returning `[]`.

Fix: check `if default is not None:` instead, matching the type
signature (`default: list[str] | None = None`) and the pattern already
used elsewhere in this file (e.g. `MutableScopeHeaders.__getitem__`).

How verified: added test_mutable_scope_headers_getall_not_found_empty_list_default,
confirmed it fails with KeyError before the fix and passes after. Full
test_datastructures/ suite (148 tests) passes. mypy and pre-commit
(ruff, unasyncd, typos) clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: chuenchen309

litestar/datastructures/headers.py

@@ -169,7 +169,7 @@ def getall(self, key: str, default: list[str] | None = None) -> list[str]:
             if header_name.decode("latin-1").lower() == name
         ]
         if not values:
-            if default:
+            if default is not None:
                 return default
             raise KeyError
         return values

tests/unit/test_datastructures/test_headers.py

@@ -156,6 +156,11 @@ def test_mutable_scope_headers_getall_not_found_default(mutable_headers: Mutable
     assert mutable_headers.getall("bar", ["default"]) == ["default"]
 
 
+def test_mutable_scope_headers_getall_not_found_empty_list_default(mutable_headers: MutableScopeHeaders) -> None:
+    """An explicit empty-list default should be returned as-is, not treated as "no default"."""
+    assert mutable_headers.getall("bar", []) == []
+
+
 def test_mutable_scope_headers_extend_header_value(
     raw_headers: "RawHeadersList", mutable_headers: MutableScopeHeaders
 ) -> None: