Account for nil schema_node in ArgumentsOfCorrectType suggestions

[emeryotopalik]

When unstructured scalar input is sent in a list to a mutation argument (see example), the schema_node in the ArgumentsOfCorrectType.suggested_field_names/2 function is nil. I accounted for the case at the source of the error, but it is totally possible that the blueprint should account for this case and the schema_node should be something other than nil. I will defer to y'all's knowledge on that though.

Mutation example:

mutation($scalarInput: CustomScalar) {
    createDog(customScalarInput: $scalarInput)
}
variables: { "scalarInput": [ {"foo": "BAR"} ] }

Stacktrace:

** (exit) an exception was raised:
    ** (UndefinedFunctionError) function nil.fields/0 is undefined
        nil.fields()
        (absinthe 1.5.4) lib/absinthe/phase/document/validation/arguments_of_correct_type.ex:125: Absinthe.Phase.Document.Validation.ArgumentsOfCorrectType.suggested_field_names/2
        (absinthe 1.5.4) lib/absinthe/phase/document/validation/arguments_of_correct_type.ex:87: anonymous fn/3 in Absinthe.Phase.Document.Validation.ArgumentsOfCorrectType.collect_child_errors/2
        (elixir 1.10.4) lib/enum.ex:3343: Enum.flat_map_list/2
        (absinthe 1.5.4) lib/absinthe/phase/document/validation/arguments_of_correct_type.ex:33: Absinthe.Phase.Document.Validation.ArgumentsOfCorrectType.handle_node/2
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:16: anonymous fn/3 in Absinthe.Blueprint.Transform.prewalk/2
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:109: Absinthe.Blueprint.Transform.walk/4
        (elixir 1.10.4) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:145: anonymous fn/4 in Absinthe.Blueprint.Transform.walk_children/5
        (elixir 1.10.4) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:114: Absinthe.Blueprint.Transform.walk/4
        (elixir 1.10.4) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:145: anonymous fn/4 in Absinthe.Blueprint.Transform.walk_children/5
        (elixir 1.10.4) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:114: Absinthe.Blueprint.Transform.walk/4
        (elixir 1.10.4) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:145: anonymous fn/4 in Absinthe.Blueprint.Transform.walk_children/5
        (elixir 1.10.4) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:114: Absinthe.Blueprint.Transform.walk/4
        (absinthe 1.5.4) lib/absinthe/blueprint/transform.ex:15: Absinthe.Blueprint.Transform.prewalk/2

[binaryseed]

Is this part necessary to recreate the bug?

[emeryotopalik]

Probably not! I'll verify.

[benwilson512]

Makes total sense, thanks!

[benwilson512]

@binaryseed do you mind looking into these CI errors?

[binaryseed]

some dialyzer error, pulling into a working branch to try and fix

[binaryseed]

Ok, fixed & merged via #1001