fix(flags): update frontend null checks to enable per-condition aggregation_group_type_index

Context

Part of #46288 (Mixed User + Group Targeting for Feature Flags).

PR #51399 adds aggregation_group_type_index to each condition set, but currently only sets the field when the value is non-null. For person-aggregated flags, the field is left absent to preserve backward compatibility with frontend code that uses !== undefined checks.

This blocks the full per-condition aggregation vision where every condition set explicitly carries its aggregation mode (including None for person-aggregated).

Problem

~10 frontend locations use !== undefined to check aggregation_group_type_index. In JavaScript, null !== undefined is true, so adding explicit null values would cause these checks to incorrectly treat person-aggregated flags as group-aggregated.

Affected locations

Fix

Replace !== undefined / != undefined with != null (which catches both null and undefined) or use explicit != null checks. Then update the Python normalization to always set the field, and backfill existing flags.

Steps

1. Fix all frontend !== undefined checks for aggregation_group_type_index to use != null
2. Update posthog/api/feature_flag.py normalization to always set the field (remove the is not None guard)
3. Backfill existing person-aggregated flags to have explicit aggregation_group_type_index: null on each condition set

Start with the listed frontend files, including relatedFeatureFlagsLogic.ts, experimentLogic.tsx, featureFlagLogic.ts, FunnelsAdvanced.tsx, and the filter and funnel logic files, then inspect posthog/api/feature_flag.py. Update the null checks, make normalization always set the field, and backfill existing person-aggregated flags so every condition set has an explicit null value.