MLFlowLogger throws a JSONDecodeError
🐛 Bug

To Reproduce
Steps to reproduce the behavior:
Code sample

from pytorch_lightning import Trainer
from pytorch_lightning.loggers import MLFlowLogger
mlflow_logger = MLFlowLogger(experiment_name="test-experiment", tracking_uri="URI_HERE")
t = Trainer(logger=mlflow_logger)
t.logger.experiment_id
throws a JSONDecodeError exception.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/envs/pl_env/lib/python3.7/site-packages/pytorch_lightning/loggers/mlflow.py", line 120, in experiment_id
    _ = self.experiment
  File "/envs/pl_env/lib/python3.7/site-packages/pytorch_lightning/loggers/base.py", line 421, in experiment
    return get_experiment() or DummyExperiment()
  File "/envs/pl_env/lib/python3.7/site-packages/pytorch_lightning/utilities/distributed.py", line 13, in wrapped_fn
    return fn(*args, **kwargs)
  File "/envs/pl_env/lib/python3.7/site-packages/pytorch_lightning/loggers/base.py", line 420, in get_experiment
    return fn(self)
  File "/envs/pl_env/lib/python3.7/site-packages/pytorch_lightning/loggers/mlflow.py", line 98, in experiment
    expt = self._mlflow_client.get_experiment_by_name(self._experiment_name)
  File "/envs/pl_env/lib/python3.7/site-packages/mlflow/tracking/client.py", line 154, in get_experiment_by_name
    return self._tracking_client.get_experiment_by_name(name)
  File "/envs/pl_env/lib/python3.7/site-packages/mlflow/tracking/_tracking_service/client.py", line 114, in get_experiment_by_name
    return self.store.get_experiment_by_name(name)
  File "/envs/pl_env/lib/python3.7/site-packages/mlflow/store/tracking/rest_store.py", line 219, in get_experiment_by_name
    response_proto = self._call_endpoint(GetExperimentByName, req_body)
  File "/envs/pl_env/lib/python3.7/site-packages/mlflow/store/tracking/rest_store.py", line 32, in _call_endpoint
    return call_endpoint(self.get_host_creds(), endpoint, method, json_body, response_proto)
  File "/envs/pl_env/lib/python3.7/site-packages/mlflow/utils/rest_utils.py", line 145, in call_endpoint
    js_dict = json.loads(response.text)
  File "/envs/pl_env/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/envs/pl_env/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/envs/pl_env/lib/python3.7/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
Expected behavior

Environment
Environment details

 - PyTorch Version (e.g., 1.0): 1.6.0
 - PyTorch Lightning Version: 0.9.0rc12
 - OS (e.g., Linux): Linux
 - How you installed PyTorch (`conda`, `pip`, source): conda
 - Build command you used (if compiling from source):
 - Python version: 3.7.7
 - CUDA/cuDNN version: Not relevant
 - GPU models and configuration: Not relevant
 - Any other relevant information: Not relevant

### Additional context

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