Check for an empty executable passed into the accelerate plugin

This was breaking at least the script module, so it would seem best
to check for it and set it to the default executable value

lib/ansible/runner/connection_plugins/accelerate.py

@@ -138,6 +138,9 @@ class Connection(object):
     def exec_command(self, cmd, tmp_path, sudo_user, sudoable=False, executable='/bin/sh'):
         ''' run a command on the remote host '''
 
+        if executable == "":
+            executable = constants.DEFAULT_EXECUTABLE
+
         if self.runner.sudo or sudoable and sudo_user:
             cmd, prompt = utils.make_sudo_cmd(sudo_user, executable, cmd)