add executable if none exists when become

this is needed as become adds commands that require shell

(cherry picked from commit 4acac55d23)

lib/ansible/playbook/play_context.py

@@ -447,7 +447,7 @@ class PlayContext(Base):
 
         if self.become:
 
-            if executable is None:
+            if not executable:
                 executable = self.executable
 
             becomecmd   = None