fix: handle incorrect string type for nested strcuts

[steveiliop56]

For some reason, when I used nested structs, so something like this:

type Yoyo struct {
    Yoyo map[string]Yaya
}

type Yaya struct {
    Foo Fooya
}

type Fooya struct {
    Bar string
}

And then an environment variable like this:

TRAEFIK_YOYO_NAME_FOO_BAR=bar

The parser would fail to walk through the struct saying that it expected a struct instead of a string. This change (although a bit junky), forces the struct type on the field if the node has children, essentially allowing the parsing to proceed and the decode to work.