gclient: fix assertion crash when a dep is set to None

This is a follow-up after https://chromium-review.googlesource.com/c/517049/

Bug: 727334, 570091
Change-Id: I49f9ad07f1b79cba547d88e93b2d2064c33e1f4b
Reviewed-on: https://chromium-review.googlesource.com/518125Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

gclient.py

@@ -651,6 +651,8 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
         ent = self.recursedeps.get(name)
         if ent is not None:
           deps_file = ent['deps_file']
+      if dep_value is None:
+        continue
       if isinstance(dep_value, basestring):
         url = dep_value
       else:

testing_support/fake_repos.py

@@ -451,6 +451,17 @@ deps = {
     'url': '%(git_base)srepo_2@%(hash)s',
   },
 }
+deps_os ={
+  'mac': {
+    'src/none_repo': None,
+  },
+  'unix': {
+    'src/none_repo': None,
+  },
+  'win': {
+    'src/none_repo': None,
+  },
+}
 hooks = [
   {
     'pattern': '.',