How to safely block French doors to nowhere
In my area, they often build homes that have an optional deck, but the deck is not built at the same time as the home. In these cases, the home builders simply build a small section of railing, and attach it directly in front of the door.  In your case, you will not be able to open the doors (since the railing will prevent it).  In the case of sliding doors, you can open the door, but will not be able to go out it.

You can buy nice looking barrier railings to put up, if you're going to leave it up for a long time. 



If it's going to be temporary, the builders around here usually just tack up a cheap section of railing.