Don't try to process empty Pandas data frame
Don't try to process empty Pandas data frame

If we get no data back from HAProxy and try to process it we die as
we receive::

Traceback (most recent call last):
  File ".../multiprocessing/process.py", line 254, in _bootstrap
    self.run()
  File ".../haproxystats/process.py", line 106, in run
    self.process_stats(incoming_dir)
  File ".../haproxystats/process.py", line 130, in process_stats
    self.sites_stats(get_files(pathname, FILE_SUFFIX_STAT))
  File ".../haproxystats/process.py", line 193, in sites_stats
    self.process_backends(data_frame)
  File ".../haproxystats/process.py", line 238, in process_backends
    as_index=False).mean()
  File ".../pandas/core/groupby.py", line 760, in mean
    return self._cython_agg_general('mean')
  File ".../pandas/core/groupby.py", line 2853, in _cython_agg_general
    new_items, new_blocks = self._cython_agg_blocks(how, numeric_only=numeric_only)
  File ".../pandas/core/groupby.py", line 2898, in _cython_agg_blocks
    raise DataError('No numeric types to aggregate')

We fix this by checking if we create a Pandas data frame object with
index length higher than zero. This commit also adds some more debugging
logging.

unixsurfer/haproxystats,unixsurfer/haproxystats,unixsurfer/haproxystats