Ignore null-Properties for ATRules

[RichardCox09]

Consider the following example:

@media all and (-ms-high-contrast: none), (-ms-high-contrast: active) {
  * {color: red}
}

The validator validates the input as:

@media all, null {
  * {
    color : red;
  }
}

Which is of course not valid CSS.

[ylafon]

Thanks for this, instead of filtering the null entries to display them, I removed them during parsing.