Deal with Empty Ballots (And Test Correct Behaviour)

[BenBals]

As described by @lukasrad02, the current algorithm for ranked choice elections throws an exception if there is an empty ballot (i.e., a voter who ranked no choice but still submitted their ballot). I've fixed this error and implemented a test to check the behaviour.

The Error

For reference: Here is where it previously broke

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ERROR: test_empty_ballots (myhpi.tests.polls.test_ranked_choice_algorithm.RankedChoiceAlgorithmTests)
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Traceback (most recent call last):
  File "/Users/beb/code/fsr/myHPI/myhpi/tests/polls/test_ranked_choice_algorithm.py", line 117, in test_empty_ballots
    self.poll.calculate_ranking(),
  File "/Users/beb/code/fsr/myHPI/myhpi/polls/models.py", line 237, in calculate_ranking
    current_votes[heapq.heappop(ballot)[1].option.pk].append(ballot)
IndexError: index out of range

[lukasrad02]

Can confirm this is working on the data set that caused the error previously.