When Impala calculates table stats, NULL count gets overridden with -1.
Number of NULLs in a table is a useful information. Even if Impala does not benefit from this information, some other tools do. Thus, not collecting this information may pose a problem for Impala users (potentially forcing them to run COMPUTE STATS elsewhere).
My suggestion would be to
- improve expression used to collect NULL count
- collect NULL count during COMPUTE STATS