Treat groups= as default

Previous incarnations of groups support left "groups=" in the
repo .config, which is now treated as "delete all the projects".
Treat empty groups configuration the same as no groups
configuration.

Change-Id: I57dab8dac55bdbf4cc181e2748cd2e4e510764f5

command.py

@@ -69,7 +69,7 @@ class Command(object):
     mp = self.manifest.manifestProject
 
     groups = mp.config.GetString('manifest.groups')
-    if groups is None:
+    if not groups:
       groups = 'default,platform-' + platform.system().lower()
     groups = [x for x in re.split('[,\s]+', groups) if x]
 

manifest_xml.py

@@ -122,7 +122,7 @@ class XmlManifest(object):
     mp = self.manifestProject
 
     groups = mp.config.GetString('manifest.groups')
-    if groups is None:
+    if not groups:
       groups = 'default'
     groups = [x for x in re.split(r'[,\s]+', groups) if x]