[context] fix treatment of None in env.

recipe_modules/context/tests/env.py

 # Copyright 2017 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 DEPS = [
   "context",
   "path",
   "step",
 ]
 
@@ skipping 17 matching lines @@
       pants = api.path['start_dir'].join('pants')
       shirt = api.path['start_dir'].join('shirt')
       with api.context(env={'FOO': api.context.Prefix(pants, shirt)}):
         api.step('env step with prefix', ['bash', '-c', 'echo $FOO'])
 
   # Can set the path of default environment variables.
   with api.context(env={'FOO': api.context.Prefix(shirt)}):
     api.step('env with default value',
              ['bash', '-c', 'echo $FOO'])
 
+  # Can cause envvars to be dropped completely.
+  with api.context(env={"PYTHONPATH": None}):
+    api.step('drop PYTHONPATH', ['bash', '-c', 'set'])
+
 
 def GenTests(api):
   yield api.test('basic')