Multi-value CSS involving 'unset' is changed into an invalid CSS

[totoguy]

Precheck

• Do a quick search and make sure a bug has not yet been reported;
• do a quick check if the bug still exists in the latest patch version;
• finally, be nice and have fun!

Environment

• clean-css version - npm ls clean-css: 4.2.1
• node.js version - node -v: 12.13.0
• operating system: Windows 10

Configuration options

var CleanCSS = require('clean-css');
new CleanCSS({
  compatibility: 'ie9',
  level: {
     2: {
        skipProperties: [
           'transition', 
           'font', 
        ],
     },
  },
  inline: false,
  returnPromise: true,
  sourceMap: this._options.sourceMap,
})

Input CSS

.some-class {
   padding: unset;
   padding-right: 30px;
}

Actual output CSS

.some-class {
   padding: unset 30px unset unset;
}

Expected output CSS

.some-class {
   padding: unset;
   padding-right: 30px;
}