PR Checklist

• [ x ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [ x ] You have signed the CLA.
• [ x ] All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/DevelopmentWorkflow.md#running-unit-tests.
• [ x ] Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the current behavior?

The CSS parser is failing for empty rules such as:

Button.-primary {
    font-size: 18;
    ;
    color: red;
}

What is the new behavior?

The CSS parser should ignore such empty rules without failing.