Add filtering for null debug handles when inserting (#1638)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1638

DelegateMappingBuilder currently filters out missing and null debug handles when creating entries from Nodes.

This diff adds the same filtering behavior when creating entries from a list of explicit debug handles.

Reviewed By: digantdesai

Differential Revision: D52859842

fbshipit-source-id: 6ab481a9c7694ee20298736da883d6fb5bc317fa