You can tweak the logic of the NAPL output view in Access to allow the "Null" values to be output, however your graphs still won't show that measurement with a LNAPL thickness of 0 (if you are graphing thickness), or that the LNAPL and water depth lines intersect (if that is what you are graphing).  Ultimately I expect that is what you will be needing to represent.

In order to get the output to show a LNAPL thickness of 0 you will need to enter a LNAPL depth that is the same value as the Water Depth (giving it an effective thickness of 0 at that point).  Then your graphs, contours, or other outputs will have the effect you are likely to be looking for.  You will want to make sure that you document that this is what you have done, either in the comments column for that record, or in the general notes section for that database.