Allow NULL mention in a candidate

[HiromuHota]

Description of the feature request

Is your feature request related to a problem? Please describe.

As far as I understand, a candidate does not allow NULL mention.
In reality, there are cases that some mention could be NULL.
Regarding the US president information extraction from Wikipedia, Let's assume we want to extract the spouse's name in addition to the president's name and place of birth.

name	place of birth	spouse's name
Barack Obama	Hawaii	Michelle Robinson
James Buchanan	Pennsylvania

James Buchanan had no spouse in his life, so the corresponding cell should be left NULL.
But, the current implementation of Fonduer does not allow a NULL mention.

Description of the solution you'd like

Allow NULL mention in a candidate.

Description of the alternatives you've considered

A mention of "" (empty string) could be an alternative.

Additional context

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