fix: gate transformers on valid non-nil destinations

[zaquestion]

This builds on #203 which attempted to provide a more flexible gating to
running transformers. However upon testing #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.IsValid 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.

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[zaquestion]

Added a test for the specific panic I alluded to in #203, I had significant trouble reproducing the case my integration tests were hitting so I ultimately settled for adding the test in the way I did as I didn't want it to be left entirely uncovered.

[zaquestion]

@imdario Just poking you again as a reminder to peek at this. Hope ya had a lovely weekend

[darccio]

@zaquestion It wasn't as pleasant as I expected. Sorry for the delay. It looks good, all tests are green and the code feels quite right.