Floki crashes for Floki.attribute on a node list with only text nodes

[ckruse]

Description

Given this fragment:

<p>test</p>

Floki crashes when calling Floki.attribute on Floki.children of the root node:

Floki.attribute(children, "class")
** (FunctionClauseError) no function clause matching in anonymous fn/2 in Floki.attribute_values/2    
    
    The following arguments were given to anonymous fn/2 in Floki.attribute_values/2:
    
        # 1
        "test"
    
        # 2
        []
    
    lib/floki.ex:540: anonymous fn/2 in Floki.attribute_values/2
    (elixir 1.10.3) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
    lib/floki.ex:537: Floki.attribute_values/2

To Reproduce

Steps to reproduce the behavior:

• Using Floki v0.26.0

• Using Elixir v1.10.3

• Using Erlang/OTP 23 [erts-11.0.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

• With this code:

   "<p>test</p>"
  |> Floki.parse_fragment!()
  |> List.first
  |> Floki.children()
  |> Floki.attribute("class")

Expected behavior

Floki should return an empty list.