Commits on May 18, 2022

1. fix: gate transformers on valid non-nil destinations …

   This builds on darccio#203 which attempted to provide a more flexible gating to
   running transformers. However upon testing darccio#203 in my own environment, I
   ran into the first panic listed below.
   
   There are a variety of errors that can happen when trying to run
   reflection on zero values:
   
   2 just from my testing of this PR
   ```
           panic: reflect: call of reflect.Value.Type on zero Value
   
           panic: reflect: call of reflect.Value.FieldByName on zero Value
   ```
   The panic specifically calls out zero values, but it's actual a more
   specific set of values which is covered by `reflect.IsValid`.
   
   I attempted to replace the check with `reflect.IsZero`, which ends up
   being too restrictive and breaks existing tests. I also attempted to
   solely use `reflect.IsZero` which is not restrictive enough and cause
   the later panic above in the tests. Thus I arrived on the combination in
   this PR which seems to strike the "right" balance.

   Zaq? Wiedmann committed May 18, 2022
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   ab6b270 View commit details

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2. add test for keeping zero values with transformer

   Zaq? Wiedmann committed May 18, 2022
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   4bed36e View commit details

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3. add test for deepMerge panic

   Zaq? Wiedmann committed May 18, 2022
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   8109749 View commit details

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