Your formula can return NA() or "", but neither is interpreted as a blank cell: one's an error value, and the other is a short text string. The marker for NA() is omitted, and the line connects points on either side of the NA(). A marker for "" is placed at zero, since Excel interprets text as zero.
You have to manually or programmatically delete the contents of a cell in order for the chart to consider it a blank cell. Or you could manually or programmatically format the connecting line that spans the non-blank cell to use "no line".
Not the bright idea you wanted, but these are the best I've ever come across.
EDIT: A recent (late 2017) enhancement to Excel 2016 treats #N/A like a blank cell. (If you have Office 365, you can toggle this setting.) Finally now we can have a formula produce a result which is plotted as a gap.