[context] fix treatment of None in env.

recipe_modules/context/tests/env.py

Index: recipe_modules/context/tests/env.py
diff --git a/recipe_modules/context/tests/env.py b/recipe_modules/context/tests/env.py
index 867e27ea9f90e8c30a6c6a1c34c46a2526bbab5b..4ba106097f8af7708ae35cca8a43b71f2dcc39e6 100644
--- a/recipe_modules/context/tests/env.py
+++ b/recipe_modules/context/tests/env.py
@@ -35,6 +35,10 @@ def RunSteps(api):
     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')