Fix the Apply Method on Changeset to Make the Apply Method Can Set Nil Value

[imansohibul]

This PR to fix the Apply method on Changeset, so rel will be able to update the column with nil value which is from the Changes.

[codecov]

Codecov Report

Merging #8 (c2c389b) into master (cb9dfb3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #8   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines          883       883           
=========================================
  Hits           883       883           

Impacted Files	Coverage Δ
changeset.go	100.00% <100.00%> (ø)

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