fix(backend): clear empty filter by header menu not stopping spinner

[ghiscoding]

• when trying to clear a filter that was empty, there is no reason to trigger a filter change
• to replicate the issue, you could go to the OData or GraphQL example and hover over a column with an empty filter, then choose "Clear Filter" from the header menu. The spinner should stop

[codecov-io]

Codecov Report

Merging #241 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   75.31%   75.33%   +0.02%     
==========================================
  Files         145      145              
  Lines        6962     6966       +4     
  Branches     2209     2211       +2     
==========================================
+ Hits         5243     5247       +4     
  Misses       1719     1719

Impacted Files	Coverage Δ
...dules/angular-slickgrid/services/filter.service.ts	100% <100%> (ø)	⬆️

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