Fixing spacing after removing stopwords

Question

I have run a code to remove stopwords, but the output has removed the space between the words.

The initial input before the stopword removal is like this:

form server happen

The word output looks like this:

formserverhappen 

Code:

#Removing stopwords

def remove_stopwords(text,nlp,custom_stop_words=None,remove_small_tokens=True,min_len=2):
    # if custom stop words are provided, then add them to default stop words list
    if custom_stop_words:
        nlp.Defaults.stop_words |= custom_stop_words
    
    filtered_sentence = [] 
    doc=nlp(text)
    for token in doc:
        
        if token.is_stop == False: 
            
            # if small tokens have to be removed, then select only those which are longer than the min_len 
            if remove_small_tokens:
                if len(token.text)>min_len:
                    filtered_sentence.append(token.text)
            else:
                filtered_sentence.append(token.text)
    # if after the stop word removal, words are still left in the sentence, then return the sentence as a string else return null 
    return '  '.join(filtered_sentence) if len(filtered_sentence)>0 else None
#creating a spaCy object. 
nlp = spacy.load("en_core_web_sm", disable=["parser", "ner"])

#removing stop-words and short words from every row
negative.ctstring=negative.ctstring.apply(lambda x:remove_stopwords(x,nlp,{"mask","mandates",}))

Picture of the code

I looked that join but I can't figure out why the change is occuring.