fix concatenate to be more robust

[christopherlovell]

https://github.com/flaresimulations/eagle_IO/blob/master/eagle_IO/eagle_IO.py#L144

quote from Will:

"...and simply wrapping the concatenate in if len(dat) != 0: else: dat = np.array([]) then you’re covered in all instances? Maybe with an optional print statement if verbose"