Running REAL_TIME_5_MIN in the first interval throws unexpected error

[blakeShalom]

At 01:39 ET I was running tests and saw unexpected errors

import gridstatus
iso = gridstatus.ISONE()
iso.get_lmp("today", market='REAL_TIME_5_MIN')
[]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../gridstatus/gridstatus/lmp_config.py", line 26, in wrapper
    return self._class_method_wrapper(func, bound_args)
  File ".../gridstatus/gridstatus/lmp_config.py", line 61, in _class_method_wrapper
    return func(*instance_args, **instance_kwargs)
  File ".../gridstatus/gridstatus/decorators.py", line 78, in wrapped_f
    df = f(**args_dict)
  File ".../gridstatus/isone.py", line 307, in get_lmp
    data = pd.concat(dfs)
  File ".../grid_env/lib/python3.8/site-packages/pandas/util/_decorators.py", line 331, in wrapper
    return func(*args, **kwargs)
  File ".../grid_env/lib/python3.8/site-packages/pandas/core/reshape/concat.py", line 368, in concat
    op = _Concatenator(
  File ".../grid_env/lib/python3.8/site-packages/pandas/core/reshape/concat.py", line 425, in __init__
    raise ValueError("No objects to concatenate")
ValueError: No objects to concatenate

We should probably raise a more user friendly exception to catch during testing in the middle of the night or if this were running on a chron every few hours.