Use types.Null instead of nil

[AlekSi]

Closes #216.

[codecov]

Codecov Report

Merging #268 (fb6882a) into main (ba59015) will decrease coverage by 0.07%.
The diff coverage is 75.63%.

@@            Coverage Diff             @@
##             main     #268      +/-   ##
==========================================
- Coverage   49.85%   49.78%   -0.08%     
==========================================
  Files         110      112       +2     
  Lines        5311     5323      +12     
==========================================
+ Hits         2648     2650       +2     
- Misses       2251     2260       +9     
- Partials      412      413       +1     

Impacted Files	Coverage Δ
internal/bson/array.go	62.74% <ø> (ø)
internal/bson/bool.go	60.00% <ø> (ø)
internal/bson/bson.go	0.00% <0.00%> (ø)
internal/bson/cstring.go	68.18% <ø> (ø)
internal/bson/datetime.go	67.85% <ø> (ø)
internal/bson/double.go	70.83% <ø> (ø)
internal/bson/int32.go	68.18% <ø> (ø)
internal/bson/int64.go	68.18% <ø> (ø)
internal/bson/null.go	0.00% <0.00%> (ø)
internal/bson/objectid.go	65.00% <ø> (ø)
... and 21 more

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