If i have a NULL value and
tooltip.pointFormat: '{series.name}<br />{point.x:%e %b, %Y} {point.y}:00: <b>{point.value:,.4f}</b>'

The tooltip will be:
ABC<br /> 01.01.2014 30:00 0.0000
but better solution would be:
ABC<br /> 01.01.2014 30:00 No value

Here an example:
http://jsfiddle.net/25MyN/20/

Something like
tooltip.nullPointFormat

would mek alot easyer.