pygrass: Avoid conversion if default parameter is an empty string

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@61213 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/python/pygrass/modules/interface/parameter.py

@@ -156,7 +156,7 @@ class Parameter(object):
         #
         # default
         #
-        if 'default' in diz:
+        if 'default' in diz and diz['default']:
             if self.multiple or self.keydescvalues:
                 self.default = [self.type(v)
                                 for v in diz['default'].split(',')]