charts: Fix numerical data that can be null

charts: Fix numerical data that can be null

The numerical consumer assumed the data received could not be null, but
it can be null.

If the test predicate passes, null values are mapped to zero.

Change-Id: If819bf29044a207581b36be1d35e846948c20650
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/89466
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>