Instructions with only the escape character should have an empty list of properties

[rcjsuen]

FROM scratch
ENV \

$ docker build .
Sending build context to Docker daemon   46.8MB
Error response from daemon: Dockerfile parse error line 2: ENV requires at least one argument

Similar to #83, the parser should not consider the ENV instruction as having any properties.

let dockerfileAst = require("dockerfile-ast");
let dockerfile = dockerfileAst.DockerfileParser.parse("ENV \\");
console.log(dockerfile.getInstructions()[0].getProperties().length);

1