fix: use empty string instead of null for root field name

[paralin]

No description provided.

[codecov]

Codecov Report

Merging #14 into master will increase coverage by 0.47%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   74.46%   74.93%   +0.47%     
==========================================
  Files          15       15              
  Lines        1590     1584       -6     
  Branches      379      378       -1     
==========================================
+ Hits         1184     1187       +3     
+ Misses        342      335       -7     
+ Partials       64       62       -2

Impacted Files	Coverage Δ
src/query-tree/query-tree.ts	90.48% <75%> (+1.84%)	✅

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[codecov]

Codecov Report

Merging #14 into master will increase coverage by 0.47%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   74.46%   74.93%   +0.47%     
==========================================
  Files          15       15              
  Lines        1590     1584       -6     
  Branches      379      378       -1     
==========================================
+ Hits         1184     1187       +3     
+ Misses        342      335       -7     
+ Partials       64       62       -2

Impacted Files	Coverage Δ
src/query-tree/query-tree.ts	90.48% <75%> (+1.84%)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d0ee36e...e8d2961. Read the comment docs.