[BUG] write_json fails for struct with zero columns

[karthikeyann]

Describe the bug
When a table has struct with zero columns, it fails to write using cudf::write_json.

Steps/Code to reproduce bug
case 1:

# write a json with struct with zero columns, sometimes nulls
json_str = """
{"A": {"B": null, "C": 1}}
{"A": {"B": null}}
{"A": {"B": {}}}
{"A": {"B": {}}}
{"A": {}}
{"A": {}}
"""
import cudf
from io import StringIO
df1 = cudf.read_json(StringIO(json_str), lines=True)
df1[["A"]].to_json('tmp', engine='cudf')

Also, another repro (different issue).
case 2:

# write a json with struct with zero columns, sometimes nulls
json_str = """
{"A": null}
{"A": null}
{"A": {}}
{"A": {}}
{}
{}
"""
import cudf
from io import StringIO
df1 = cudf.read_json(StringIO(json_str), lines=True)
df1[["A"]].to_json('tmp', engine='cudf')

Expected behavior
using pandas engine, df1[["A"]].to_json() produces

case 1: '{"A":{"0":{"B":null,"C":1},"1":{"B":null,"C":null},"2":{"B":{},"C":null},"3":{"B":{},"C":null},"4":{"B":null,"C":null},"5":{"B":null,"C":null}}}'
case 2: '{"A":{"0":null,"1":null,"2":{},"3":{},"4":null,"5":null}}'

Additional context
@GregoryKimball found the issue while working with synthesized json files.