Handle empty buffer returned from Edge

ObsPy's waveserver.py (and associated methods) do not handle an empty
buffer returned from Edge's earthworm server. In short, they generate
a None value instead of an empty byte string as one might expect (and
they clearly did expect, since they attempted `len(dat)` where dat
was expected to be a byte string, but was a None object if/when Edge
had no data. This fix employs a try/except clause to generate an
empty string if the waveserver client fails with a TypeError, as is
the case when Edge returns an empty buffer.