trace_processor: fix nulls in non-string columns

In either the case where the whole column was nulls or we had nulls
mixed with numeric types, we were unable to accurately represent the
output rows.

Change the trace processor to keep keep track of the nullity of each row
as well as adding an "unknown" type for columns which are all nulls.

Change-Id: I610be0ed5a6603a3d7572447d2e0dad2727f5252