trx fails when saving empty trx files.

[skoudoro]

TRX should not fail to save empty tractogram.

here is the error:

    def _create_trx_from_pointer(
        header: dict,
        dict_pointer_size: dict,
        root_zip: Optional[str] = None,
        root: Optional[str] = None,
    ) -> Type["TrxFile"]:
        """After reading the structure of a zip/folder, create a TrxFile
    
        Keyword arguments:
            header -- A TrxFile header dictionary which will be used for the
                new TrxFile
            dict_pointer_size -- A dictionary containing the filenames of all
                the files within the TrxFile disk file/folder
            root_zip -- The path of the ZipFile pointer
            root -- The dirname of the ZipFile pointer
    
        Returns:
            A TrxFile constructer from the pointer provided
        """
        # TODO support empty positions, using optional tag?
        trx = TrxFile()
        trx.header = header
        positions, offsets = None, None
        for elem_filename in dict_pointer_size.keys():
            if root_zip:
                filename = root_zip
            else:
                filename = elem_filename
    
            folder = os.path.dirname(elem_filename)
            base, dim, ext = _split_ext_with_dimensionality(elem_filename)
            if ext == ".bit":
                ext = ".bool"
            mem_adress, size = dict_pointer_size[elem_filename]
    
            if root is not None:
                # This is for Unix
                if os.name != 'nt' and folder.startswith(root.rstrip("/")):
                    folder = folder.replace(root, "").lstrip("/")
                # These three are for Windows
                elif os.path.isdir(folder) and os.path.basename(folder) in ['dpv', 'dps', 'groups']:
                    folder = os.path.basename(folder)
                elif os.path.basename(os.path.dirname(folder)) == 'dpg':
                    folder = os.path.join('dpg', os.path.basename(folder))
                else:
                    folder = ''
    
            # Parse/walk the directory tree
            if base == "positions" and folder == "":
                if size != trx.header["NB_VERTICES"] * 3 or dim != 3:
>                   raise ValueError("Wrong data size/dimensionality.")
E                   ValueError: Wrong data size/dimensionality.

[arokem]

What should the behavior be here? Refuse to save and raise a reasonable error? Or create an empty file (with 0-byte arrays)?