Fix emit of explicit keys

[biojppm]

• fix emit of explicit keys when indented:

  fixed:
    ? explicit key
    : value
  previously:
    ? explicit key
  : value   # this was not indented

Also:

• fix parser reset for full reuse (m_doc_empty was not resetted), which would cause problems under specific scenarios in subsequent reuse.
• add tests ensuring val_is_null() bug #471 is fixed
• add tests ensuring crash parsing explicit keys/empty values #259 is fixed

Closes #471 (likely fixed in #489)
Closes #259

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.32%. Comparing base (d4457b9) to head (6291900).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
+ Coverage   97.30%   97.32%   +0.01%     
==========================================
  Files          34       34              
  Lines       11322    11325       +3     
==========================================
+ Hits        11017    11022       +5     
+ Misses        305      303       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.