Default fmt of IStructure.to doesn't agree with docstring

[DanielYang59]

As per the IStructure.to docstring:

pymatgen/src/pymatgen/core/structure.py

Lines 2920 to 2932 in b5d03d7

	def to(self, filename: PathLike = "", fmt: FileFormats = "", **kwargs) -> str:
	"""Output the structure to a file or string.
	Args:
	filename (PathLike): If provided, output will be written to a file. If
	fmt is not specified, the format is determined from the
	filename. Defaults is None, i.e. string output.
	fmt (str): Format to output to. Defaults to JSON unless filename
	is provided. If fmt is specifies, it overrides whatever the
	filename is. Options include "cif", "poscar", "cssr", "json",
	"xsf", "mcsqs", "prismatic", "yaml", "yml", "fleur-inpgen", "pwmat",
	"aims".
	Non-case sensitive.

The default fmt should be json when filename is not provided.

However:

from pymatgen.core import Structure, Lattice


structure = Structure(
    lattice=Lattice.cubic(3),
    species=("Fe", "Fe"),
    coords=((0, 0, 0), (0.5, 0.5, 0.5)),
)

structure.to()

Would throw an error as the fmt is empty string instead of JSON:

Traceback (most recent call last):
  File "/Users/yang/developer/pymatgen/debug/test_structure_to.py", line 10, in <module>
    structure.to()
    ~~~~~~~~~~~~^^
  File "/Users/yang/developer/pymatgen/src/pymatgen/core/structure.py", line 3028, in to
    raise ValueError(f"Format not specified and could not infer from {filename=}")
ValueError: Format not specified and could not infer from filename=''

pymatgen/src/pymatgen/core/structure.py

Lines 3026 to 3029 in b5d03d7

	else:
	if fmt == "":
	raise ValueError(f"Format not specified and could not infer from {filename=}")
	raise ValueError(f"Invalid {fmt=}, valid options are {get_args(FileFormats)}")