Fix: Prevent error if property block has no value (level 0)

[clarkdave]

The tokenizer will parse a property like width: ;, but the property helper for level 0 was not accounting for properties without a value and would throw a TypeError if the value was missing.

For example, the following would throw an unhandled error:

const CleanCSS = require('clean-css')
const cleanCss = new CleanCSS({ level: 0 })

cleanCss.minify('a{color:#f00;font-weight:;background:red}')
// TypeError: Cannot read property '0' of undefined
//   at property (./clean-css/lib/writer/helpers.js:80:33)

With this change, the property helper now checks for a missing property value before trying to access it.

[jakubpawlowicz]

Good stuff @clarkdave, I'm gonna merge it.

[clarkdave]

Thanks @jakubpawlowicz !