allow empty SaQC

for convenience we should allow creation of a empty SaQC object, because one can always do something like this:

import saqc
qc = saqc.SaQC(pd.DataFrame(columns=['dummy'])).tools.drop('dummy')

with

>>> qc._data
Empty DictOfSeries
Columns: []

>>> qc._flags
Empty Flags
Columns: []