Solution : https://service.sap.com/sap/support/notes/794006 (SAP Service marketplace login required)
Key words : 
basic key figure, -digit operators o1, correct null values, key figure, -digit operators, null values, existing values, system displays, empty cell, terms query
Related Notes : 
       
| 1512632 | Sparse data and restricted key figures in WebI reports | 
| 1296360 | Unit or currency is not displayed | 
| 1291158 | Exception aggregation and mixed units or currencies | 
| 1094885 | Error message BRAIN A258 with addition of null value | 
| 995494 | Null value with operators %A and %_Aa | 
| 994194 | Formulas with Boolean operators display only zeros | 
| 984133 | The operators NDIV0 and NOERR delete unit and currency | 
| 763338 | SAPBWNews BW Support Package 11 NW'04 stack 11 RIN | 
| 723262 | SAPBWNews BW 3.1 SP Content 20 | 
| 723257 | SAPBWNews BW 3.0B Support Package 26 |