gclient flatten: fix an issue with an deps_os entry getting added to deps

The previous code did not distinguish between None and [] value for orig_deps.

This could be triggered by not having any deps, but some entries in deps_os,
in which case these deps_os entries would get added by mistake to deps.

Bug: 570091
Change-Id: Id5b41e37ea717d5d00404694ad5bfce8df9da934
Reviewed-on: https://chromium-review.googlesource.com/558349
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

gclient.py

@@ -827,12 +827,13 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
     for dep in deps_to_add:
       if dep.verify_validity():
         self.add_dependency(dep)
-    for dep in (orig_deps_to_add or deps_to_add):
+    for dep in (orig_deps_to_add if orig_deps_to_add is not None
+                else deps_to_add):
       self.add_orig_dependency(dep)
     self._mark_as_parsed(
         [Hook.from_dict(h, variables=self._vars) for h in hooks],
         orig_hooks=[Hook.from_dict(h, variables=self._vars)
-                    for h in orig_hooks or hooks])
+                    for h in (orig_hooks if orig_hooks is not None else hooks)])
 
   def findDepsFromNotAllowedHosts(self):
     """Returns a list of depenecies from not allowed hosts.