Allow for quoted empty strings with double-quotes

[RomanHotsiy]

Minimal reproducible sample:

var postcss = require('postcss');
var conditionals = require('postcss-conditionals');
 
var works = `
  @if '' == '' {
    success {}
  }
`;

// this works
var output = postcss()
  .use(conditionals)
  .process(works)
  .css;

console.log(output) // success {}

var fails = `
  @if "" == "" {
    success {}
  }
`;

// this trows error
var output2 = postcss()
  .use(conditionals)
  .process(fails)
  .css;

/*
CssSyntaxError {
     column: 3,
     line: 2,
     message: "postcss-conditionals: <css input>:2:3: Failed to parse expression",
     name: "CssSyntaxError",
     plugin: "postcss-conditionals",
     reason: "Failed to parse expression",
     source: "\n @if \"\" == \"\" {\n success {}\n }\n"
     ...
}
*/```