internal(metrics): tag session resolutions that null inherited sessions (#4704)

## Description

- Previously, we added metrics for sessions usage in
https://github.com/inngest/inngest/pull/4640
- Those metrics did not track usage of nulls
- We add a new metric tag to see when nulls are used
- EXE-2142

## Release note

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notes. -->
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## Migration note

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>