Support parsing of digest provided with an optional empty tag

[rcjsuen]

FROM node:@sha256:613685c22f65d01f2264bdd49b8a336488e14faf29f3ff9b6bf76a4da23c4700

This Dockerfile cannot be built but the tag has been declared (as the empty string) so we should return it. Instead we are returning null. Looks like this case was missed while implementing #112.

const lib = require("dockerfile-ast");
const content = "FROM node:@sha256:613685c22f65d01f2264bdd49b8a336488e14faf29f3ff9b6bf76a4da23c4700";
const instruction = lib.DockerfileParser.parse(content).getInstructions()[0];
console.log(instruction.getImageTag());
console.log(instruction.getImageTagRange());

null
null