fix: add an empty array fallback for fields with multiple documents type

[pdrvsky]

By an accident I've found a bug which was causing a query to fail. When you create a field with a type of multiple documents and then publish a post without any value added it will fail on trying to access map method on null value. The fix was to simply provide an empty array fallback in PrimeFieldDocument class.

[codecov]

Codecov Report

Merging #300 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #300   +/-   ##
=======================================
  Coverage   79.34%   79.34%           
=======================================
  Files         108      108           
  Lines        2184     2184           
  Branches      169      153   -16     
=======================================
  Hits         1733     1733           
- Misses        432      433    +1     
+ Partials       19       18    -1

Impacted Files	Coverage Δ
...ges/prime-field-document/src/PrimeFieldDocument.ts	51.06% <0%> (ø)	⬆️
packages/prime-core/src/utils/connect.ts	91.66% <0%> (ø)	⬆️

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[birkir]

Thank you @pdrvsky !