Fix 525: ff.RawVariation not working with default value as nil

[thomaspoignant]

Description

ff.RawVariation not working with default value as nil.
This PR allow to have nil as default value.

Changes include

• Bugfix (non-breaking change that solves an issue)

Closes issue(s)

Resolve #525

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #526 (8a0b42b) into main (3108ae3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #526   +/-   ##
=======================================
  Coverage   90.37%   90.37%           
=======================================
  Files          51       51           
  Lines        2202     2202           
=======================================
  Hits         1990     1990           
  Misses        168      168           
  Partials       44       44           

Impacted Files	Coverage Δ
variation.go	98.13% <100.00%> (ø)

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