function blankLine( width, fill, borderLeft, borderRight ) {
	var line;
	if ( borderRight.length === 0 ) {
		line = '';
	} else {
		line = repeat( fill, width );
	}
	return borderLeft + line + borderRight;
}
Returns a blank slide line.

@private
@param {PositiveInteger} width - slide width
@param {string} fill - fill character
@param {string} borderLeft - left border
@param {string} borderRight - right border
@returns {string} blank slide line