How to remove the empty list in the Series, and at the same time change the non-empty list into a string form?

Question

The following is an example

df=pd.DataFrame({\'AAA\',\'BBBB\',\'CCC\'},columns=[\'fruit\'])
a=pd.Series({\'0\':\'A\',\'1\':\'B\',\'2\':\'C\'})
dict_a={}
for x in