Set Analysis doesn't work on a field without value

[#1]

I'm loading data from a database, but I don't know beforehand wich fields will contain values.

Let's say my database contains the equivalent of:

Data:
LOAD * INLINE [
Country, Value, DummyField
DUMMY, DUMMY, DUMMY
];
CONCATENATE (Data)
LOAD * INLINE [
Country, Value
USA, 10
France, 20
Japan, 30 
];

If I load all the data:

NewData:
NoConcatenate LOAD *
RESIDENT Data
;
DROP TABLE Data;

And I display a graph:

the Set Analysis "DummyField={'BLABLA'}" works (I don't have data with DummyField equals to "BLABLA").

BUT

If I load only a part of the data, that makes the field "DummyField" always empty:

NewData:
NoConcatenate LOAD *
RESIDENT Data
WHERE Country <> 'DUMMY'
;
DROP TABLE Data;

The same graph displays:

We notice that the Set Analysis "DummyField={'BLABLA'}" doesn't work: I don't have lines with DummyField equals to "BLABLA", but I still have a result.

[#2]

A workaround is to load every fields one by one with "if(len(my_field) > 0, my_field, 'N/A') AS my_field", but it's a pain in the foot.