What happens if at the beginning of my turn I have no Beans left in my hand to plant because I traded them all away on other people's turns?
The official Rio Grand Games rules cover this eventuality (page 4) the last sentence under Plant Bean Cards
I have no access to the Amigo Games version, or the German version of the rules. The Game Cabinet site does have a translation:
You just skip the first step, since you are unable to fulfill the conditions.