fix: gate transformers on valid non-nil destinations

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.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.

merge.go

@@ -79,7 +79,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		visited[h] = &visit{addr, typ, seen}
 	}
 
-	if config.Transformers != nil && !isEmptyValue(dst) {
+	if config.Transformers != nil && !isReflectNil(dst) && dst.IsValid() {
 		if fn := config.Transformers.Transformer(dst.Type()); fn != nil {
 			err = fn(dst, src)
 			return