CSS code hints improvements

__Select the initial hint on backspace key press__

Problem: When you input "col", the code hint plugin will present hints like "color","column-count",etc, with the initial item "color" selected. But, after you press backspace key once, nothing is selected on the hints list.

This patch set the `selectInitial = true` on `implicitChar === null`, making the first hint selected on backspace key press.

__Clear hints when nothing's left__

Problem: When you input "co", then press backspace twice to delete the CSS property, the code hints list remains open with hints like "align-content","align-items",etc.

This patch fixed the problem by returning `null` for the hints list when `this.info.offset === 0`.