fix the bug reflect on zero Value

[ninghejun]

fix the bug ：reflect: call of reflect.Value.Type on zero Value

if the slice is nil， or len==0， will be panic.
the case:

type Foo struct{
	Bar[]int
}

func TestGoFunk(t *testing.T) {
	foo := []Foo{}
	bars := funk.Get(foo, "Bar") //panic !!
	fmt.Print(bars)
}

[thoas]

Thank you!