Template rendering fails when using an undefined variable

[j-vizcaino]

Summary

Rendering fails when using an undefined variable.
Previous implementation was replacing the value with <no value>.
For more information, see https://devdocs.io/go/text/template/index#Template.Option

Closing issues

Fixes #25

Checklist

• Have you fmt your code locally prior to submission (orbit run fmt)?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally (orbit run ci)?
• I have squashed any insignificant commits

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@28db184). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #26   +/-   ##
=========================================
  Coverage          ?   93.11%           
=========================================
  Files             ?        7           
  Lines             ?      218           
  Branches          ?        0           
=========================================
  Hits              ?      203           
  Misses            ?        7           
  Partials          ?        8

Impacted Files	Coverage Δ
app/generator/generator.go	89.74% <100%> (ø)

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