SeqIO.GenBank: survive empty feature qualifiers

When a GenBank file contains an empty qualifier for a key that already
contained a qualifier, the `feature_qualifier()` function fails with

AttributeError: 'NoneType' object has no attribute 'replace'

because the code doesn't return from the if value is None check designed
to catch this case.
This patch fixes the AttributeError by just returning.

Signed-off-by: Kai Blin <kai.blin@biotech.uni-tuebingen.de>
GenBank: Don't die parsing files with too short feature qualifiers

I just stumbled over an input file that didn't specify a locus for some
misc_feature in production. This caused the GenBank parser to die with
an IndexError. As that's not the greatest error message to return to
library users, just handle and ignore invalid feature qualifiers.

Signed-off-by: Kai Blin <kai.blin@biotech.uni-tuebingen.de>

Conflicts:

	Tests/output/test_GenBank
	Tests/test_GenBank.py