fixes failed styleguide generation when section modifier has no markup

[aslansky]

If a section had modifiers without markup like:

// The standard call to action button
//
// :hover             - Subtle hover highlight.
// :active            - Active state of the button
// .disabled          - Dims the button to indicate it cannot be used.
// [disabled]         - Dims the button to indicate it cannot be used.
// .icon-only         - Hides text of button
//
// Styleguide 2.0

The styleguide generation failed with:
TypeError: Cannot call method 'indexOf' of null