fix(fields): api can yield an empty string for keytext field in certain cases

[lihbr]

…in cases

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

See: https://200629-sms-hoy.cdn.prismic.io/api/v2/documents/search?ref=Yg94MRIAAC0AUH0E&q=%5B%5Bat(document.type%2C+%22kitchen_sink%22)%5D%5D

Checklist:

• [ ] My change requires an update to the official documentation.
• [ ] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [ ] All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Merging #26 (3ffb789) into master (60e0283) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           15        17    +2     
=========================================
+ Hits            15        17    +2     

Impacted Files	Coverage Δ
src/fields.ts	100.00% <ø> (ø)
src/index.ts	100.00% <0.00%> (ø)

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