Fix 1.3 imperative if warnings

[joshprice]

warning: the variable "acc" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/lang/ast/parallel_visitor.ex:30

warning: the variable "accumulator" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/validation/rules/fields_on_correct_type.ex:27

warning: the variable "message" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/validation/rules/fields_on_correct_type.ex:91

warning: the variable "accumulator" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/lang/ast/type_info_visitor.ex:150

warning: the variable "accumulator" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/lang/ast/type_info_visitor.ex:171

warning: the variable "acc" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/validation/rules/no_fragment_cycles.ex:77

warning: the variable "acc" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/graphql/validation/rules/no_fragment_cycles.ex:77

warning: function GraphQL.Lang.AST.TypeInfo.nullable_type/2 is undefined or private
  lib/graphql/lang/ast/type_info_visitor.ex:127