net worth: set to 0 if it's empty for charts
net worth: set to 0 if it's empty for charts

fix #351

beancount/fava,aumayr/beancount-web,aumayr/beancount-web,aumayr/beancount-web,yagebu/fava,yagebu/fava,aumayr/beancount-web,beancount/fava,beancount/fava,yagebu/fava,beancount/fava,yagebu/fava,beancount/fava,yagebu/fava