TypeError thrown if getVariables() called on invalid LABEL

[rcjsuen]

LABEL abc

While the above Dockerfile is not valid since a LABEL instruction must have a value associated with it, calling getVariables() on it shouldn't result in an error.

import { DockerfileParser } from 'dockerfile-ast';

const content = "LABEL abc";

let dockerfile = DockerfileParser.parse(content);
let variables = dockerfile.getInstructions()[0].getVariables();
console.log(variables.length);

$ node test.js
dockerfile-ast\out\src\instructions\label.js:17
            if (value.length > 2 && value.charAt(0) === '\'' && value.charAt(value.length - 1) === '\'') {
                      ^

TypeError: Cannot read property 'length' of null
    at Label.getVariables (dockerfile-ast\out\src\instructions\label.js:17:23)
    at Object.<anonymous> (dockerfile-ast\out\src\tmp.js:7:49)
    at Module._compile (module.js:643:30)
    at Object.Module._extensions..js (module.js:654:10)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
    at Function.Module.runMain (module.js:684:10)
    at startup (bootstrap_node.js:187:16)
    at bootstrap_node.js:608:3