fix(tree grid): assert node children is defined before spreading

[yggg]

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Short description of what this resolves:

Spread on undefined throws in ES2015. Closes #1849

[codecov]

Codecov Report

Merging #1898 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1898      +/-   ##
==========================================
- Coverage   84.08%   84.07%   -0.02%     
==========================================
  Files         234      234              
  Lines        7723     7724       +1     
  Branches      679      680       +1     
==========================================
  Hits         6494     6494              
- Misses       1031     1032       +1     
  Partials      198      198

Impacted Files	Coverage Δ
...ponents/tree-grid/data-source/tree-grid.service.ts	60% <0%> (-2.5%)	⬇️