Measuring originality of a person on a sentence completion task
First, I am a bit worried about your comment:


  (a) They are told to finish a sentence differently of others.


Are they given any other instructions? Do the answers have to "make sense" somehow? What's to stop someone from answering "yesterday I bought a stomach" for instance? That would certainly be unique. You do say they had to have answers they thought were "suitable" but, still, that's a huge range! Maybe not "stomach" but what about "heart lung machine"?

Second, I think you have to precisely define what you mean by "creative". You say (also in a comment) that "least used words mean higher creativity" but I am not so sure that I agree.

But, if you want the least used word and you want to account for the "no answers" then one way is:


All no answers get a 0 score on creativity.
Call the number of answers (excluding those with no answers) $N$
For each answer, call the number of people who gave that answer $Q_i$ where i ranges from 1 to the number of different answers. 
The creativity score of each answer is $N/Q_i$


The key is step 4. There are certainly other formulas you could use. Which one to choose depends on what you want the distribution of creativity to look like and whether you thing "only person with this answer" is twice as creative as "one of two people with this answer".

But, again, I'd hesitate to call this creativity.