when span > 2 ,open execl file will be error

[zhu1fei]

json-excel-converter/json_excel_converter/xlsx/__init__.py

Line 179 in add62a4

if cell_data.columns > 1 or cell_data.span > 1:

when span >2 , merge_range(L197) will merge multiple times, generated execl can't open.

this json data convert to execl will be open with a error.
[
{
"col1": "val1",
"col2": [
{
"col21": {"col31": "val3"}
},
{
"col21": {}
}
]
}
]

fix method:
json_excel_converter/xlsx/init.py#L179：
if cell_data.columns > 1 or cell_data.span > 1:
change to ：
if (cell_data.columns > 1 or cell_data.span > 1) and cell_data.value != '':