Handle case when Plot has no crops

[alexgleason]

If a plot has no crops an order cannot be placed.

I'm wondering if we need to rethink how crops work, per #46

[marykatefain]

I feel like this is actually nearly under control now. The user has 3 things they can do at this point:

1. Toggle the "harvest everything" button
2. Click the "add crops" button and fill in their plot before returning to creating an order
3. Leave crops blank, but include a comment detailing their request

Any of these should give the picker the info they need. 1 and 2 currently work, but 3 returns an error to select the crops. I think we need to just allow them to submit an order with no crops at this point.

[alexgleason]

You're right. We'll just need to ensure that the form responds reasonably depending on what the user inputs before hitting submit in #62.