Add validator that checks if any default gift card configuration exists

[loevgaard]

Fixes #211

[codecov[bot]]

Codecov Report

Base: 47.13% // Head: 47.13% // No change to project coverage :thumbsup:

Coverage data is based on head (2ed81d3) compared to base (2ed81d3). Patch has no changes to coverable lines.

:exclamation: Current head 2ed81d3 differs from pull request most recent head 589302e. Consider uploading reports for the commit 589302e to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.12.x #240 +/- ## ========================================= Coverage 47.13% 47.13% Complexity 565 565 ========================================= Files 106 106 Lines 1765 1765 ========================================= Hits 832 832 Misses 933 933 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Setono). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Setono)

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