fix(fsspec): handle zero-byte files in __len__ (#3353)

## Summary

Fix `FsspecInputFile.__len__` and `FsspecOutputFile.__len__` so
zero-byte files return `0` instead of being treated as missing metadata.

Both methods previously used truthiness checks on
`object_info.get(...)`, which caused valid sizes like `0` to fall
through to the runtime error path.

## Changes

- check for `Size` key presence explicitly
- check for `size` key presence explicitly
- add a regression test covering zero-byte lengths for both metadata key
variants

## Verification

- `python -m pytest tests/io/test_fsspec.py -k zero_length_of_file -q`

Author: fallintoplace

pyiceberg/io/fsspec.py

@@ -335,10 +335,10 @@ def __init__(self, location: str, fs: AbstractFileSystem):
     def __len__(self) -> int:
         """Return the total length of the file, in bytes."""
         object_info = self._fs.info(self.location)
-        if size := object_info.get("Size"):
-            return size
-        elif size := object_info.get("size"):
-            return size
+        if "Size" in object_info:
+            return object_info["Size"]
+        elif "size" in object_info:
+            return object_info["size"]
         raise RuntimeError(f"Cannot retrieve object info: {self.location}")
 
     def exists(self) -> bool:
@@ -379,10 +379,10 @@ def __init__(self, location: str, fs: AbstractFileSystem):
     def __len__(self) -> int:
         """Return the total length of the file, in bytes."""
         object_info = self._fs.info(self.location)
-        if size := object_info.get("Size"):
-            return size
-        elif size := object_info.get("size"):
-            return size
+        if "Size" in object_info:
+            return object_info["Size"]
+        elif "size" in object_info:
+            return object_info["size"]
         raise RuntimeError(f"Cannot retrieve object info: {self.location}")
 
     def exists(self) -> bool:

tests/io/test_fsspec.py

@@ -146,6 +146,20 @@ def test_fsspec_getting_length_of_file(fsspec_fileio: FsspecFileIO) -> None:
     fsspec_fileio.delete(output_file)
 
 
+@pytest.mark.parametrize("size_key", ["Size", "size"])
+def test_fsspec_getting_zero_length_of_file(size_key: str) -> None:
+    """Test getting zero-byte lengths from object metadata."""
+    location = "s3://warehouse/empty-file"
+    fs = mock.Mock(spec=AbstractFileSystem)
+    fs.info.return_value = {size_key: 0}
+
+    output_file = fsspec.FsspecOutputFile(location=location, fs=fs)
+    assert len(output_file) == 0
+
+    input_file = fsspec.FsspecInputFile(location=location, fs=fs)
+    assert len(input_file) == 0
+
+
 @pytest.mark.s3
 def test_fsspec_file_tell(fsspec_fileio: FsspecFileIO) -> None:
     """Test finding cursor position for an fsspec file-io file"""