IsValueGetter is false for Nullables

[Romanx]

This is because nullable is technically a struct (which is a value type) but is able to have no value for its inner and so be 'null

fixes #116

[codecov]

Codecov Report

Merging #132 (3e62679) into master (d8462ea) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 3e62679 differs from pull request most recent head d19603c. Consider uploading reports for the commit d19603c to get more accurate results

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   93.06%   93.06%           
=======================================
  Files          76       76           
  Lines        2121     2121           
  Branches      380      380           
=======================================
  Hits         1974     1974           
  Misses         77       77           
  Partials       70       70           

Impacted Files	Coverage Δ
...rc/Stubble.Compilation/Contexts/CompilerContext.cs	97.08% <100.00%> (ø)
...erers/TokenRenderers/InterpolationTokenRenderer.cs	100.00% <100.00%> (ø)

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