SINQ/SEA: Fix import error due to None value

If the environment variable isn't set a None value is returned. Now an
empty string will be returned

Fixes: #4882

Change-Id: I3e319daebf10e2d8ea3ee00f6094e6896e54f51b
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/34234
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
Reviewed-by: Jens Krueger <jens.krueger@tum.de>

frappy_psi/sea.py

@@ -74,7 +74,7 @@ SERVICE_NAMES = {
 
 SEA_DIR = expanduser('~/sea')
 seaconfdir = os.environ.get('FRAPPY_SEA_DIR')
-if not exists(seaconfdir):
+if seaconfdir is None or not exists(seaconfdir):
     for confdir in generalConfig.confdir.split(os.pathsep):
         seaconfdir = join(confdir, 'sea')
         if exists(seaconfdir):