Make second argument in useFieldArray optional

[Yankovsky]

Fixes #104

[codecov]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #105   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines          57     57           
  Branches       10     10           
=====================================
  Hits           57     57

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[erikras]

Published in v3.1.1.