Filling missing values for Embedded List in Python3
According to the suggestion, @bkshi gave to me I come up with a solution here below:

Also since texts_to_sequences() function convert my list to sequences starting from 1, I could use pad_sequence() and use 0 instead of a string value. 

This solution satisfies my requirements so I used a number as padding instead of a string value.  

import pandas as pd
from keras.preprocessing.text import Tokenizer
from keras.preprocessing.sequence import pad_sequences

my_list = [['the first line'],
       ['the 2nd line'],
       ['the 3r line'],
       ['the 4th line'],
       ['the 5th line'],
       ['the'],
       ['the 5th line, this is']]
max_features = 10 #how many unique words you're using

tokenizer = Tokenizer(num_words=max_features)
tokenizer.fit_on_texts(my_list)
my_list = tokenizer.texts_to_sequences(my_list)
my_list = pad_sequences(train_X, maxlen=None, dtype='int32', padding='post', truncating='post', value=0.0)