Add Sighting.summary default value

[clenk]

See also #507.

[codecov-commenter]

Codecov Report

Merging #509 (a95a463) into master (646aaa3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #509   +/-   ##
=======================================
  Coverage   87.12%   87.12%           
=======================================
  Files         151      151           
  Lines       17702    17702           
=======================================
  Hits        15423    15423           
  Misses       2279     2279           

Impacted Files	Coverage Δ
stix2/v21/sro.py	57.89% <ø> (ø)

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