fix: adding dataset hierarchical item shouldn't cause scroll flickering

[ghiscoding]

• calling datasetHierarchical setter shouldn't cause scroll flickering, this was caused a call to the DataView setItems with an empty array which in terms always scroll back to top and that was meant to clear the dataset before recreating it, however this is certainly not needed since calling the datasetHierarchical setter will then trigger a new sort which will indirectly call DataView setItems anyway
• it might also fix issue Tree scrolls and flickers on frequent update on tree data #1201, I'm not entirely sure since the user never mentioned which type of Tree Data was used (there's 2 types of Tree that exists)

[codecov]

Codecov Report

Merging #1229 (4c5864f) into master (bdaf568) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1229   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           11        11           
  Lines          919       916    -3     
  Branches       302       299    -3     
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- Hits           919       916    -3     

Files Changed	Coverage Δ
...lickgrid/components/angular-slickgrid.component.ts	100.00% <100.00%> (ø)