Keep warning about unversioned trees until removed

Restore previous behavior and keep emitting a warning when an existing
gclient entry is not longer part of the client. Additionally, suggest
the usage of `gclient sync -D` to automatically remove them.

Change-Id: I62d4662ae4d0886d340230019419b68debffc5ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1483031
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
diff --git a/gclient_scm.py b/gclient_scm.py
index e102e80..c9c26bd 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -222,7 +222,8 @@
 
   def __init__(self, url=None, *args, **kwargs):
     """Removes 'git+' fake prefix from git URL."""
-    if url.startswith('git+http://') or url.startswith('git+https://'):
+    if url and (url.startswith('git+http://') or
+                url.startswith('git+https://')):
       url = url[4:]
     SCMWrapper.__init__(self, url, *args, **kwargs)
     filter_kwargs = { 'time_throttle': 1, 'out_fh': self.out_fh }