[BUG] NotImplemented error for empty columns

[galipremsagar]

Describe the bug
There is a newly occurring error in dask_cudf with empty columns, where a max operation would result in NotImplemented error.

Steps/Code to reproduce bug

>>> import dask
>>> import pandas as pd
>>> df = pd.DataFrame({'src':[], 'dst':[], 'value':[]})
>>> import cudf
>>> gdf = cudf.from_pandas(df)
>>> import dask_cudf
>>> ddf = dask_cudf.from_cudf(gdf, npartitions=1)
>>> ddf
<dask_cudf.DataFrame | 1 tasks | 1 npartitions>
>>> max_verts = ddf[['src', 'dst']].max().max().compute()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/base.py", line 288, in compute
    (result,) = compute(self, traverse=False, **kwargs)
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/base.py", line 571, in compute
    results = schedule(dsk, keys, **kwargs)
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/threaded.py", line 79, in get
    results = get_async(
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/local.py", line 507, in get_async
    raise_exception(exc, tb)
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/local.py", line 315, in reraise
    raise exc
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/local.py", line 220, in execute_task
    result = _execute_task(task, data)
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/core.py", line 119, in _execute_task
    return func(*(_execute_task(a, cache) for a in args))
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/utils.py", line 37, in apply
    return func(*args, **kwargs)
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/dask/dataframe/core.py", line 6931, in _reduction_chunk
    return o.to_frame().T if is_series_like(o) else o
  File "/nvme/0/pgali/envs/cudfdev/lib/python3.8/site-packages/cudf/core/dataframe.py", line 3548, in transpose
    libcudf.transpose.transpose(self),
  File "cudf/_lib/transpose.pyx", line 43, in cudf._lib.transpose.transpose
NotImplementedError: Cannot transpose string columns
>>>