expr.AsBool() not working in this case

[Levi-Wu-777]

In this case, I do expr.Compile with expr.AsBool(), and expect expr.Run will return output == false and err == nil.
But instead, it return output == nil and err == nil, which violate the expr.AsBool() command.

import (
	"github.com/expr-lang/expr"
	"github.com/stretchr/testify/assert"
)

func Test_expr(t *testing.T) {
	program, err = expr.Compile("varNotExist", expr.AllowUndefinedVariables(), expr.AsBool())
	assert.Nil(t, err)
	output, err = expr.Run(program, map[string]interface{}{})
	assert.Nil(t, err)
	assert.Equal(t, nil, output)
}