[2/n][Compute Meta] support analysis for null flag features
Summary:
## TLDR
Support using NaN default value for missing dense features in RawInputProcessor for DPER2. In preparation for subsequent support for null flag features in compute meta. For train_eval this is already supported in DPER3 and we do not plan to support this in DPER2 train eval.
Differential Revision: D22439142
fbshipit-source-id: 99ae9755bd41a5d5f43bf5a9a2819d64f3883005
