[Refactor] Simplify severity counting in countOffenseTypes

[github-actions]

WHY are these changes introduced?

countOffenseTypes in packages/theme/src/cli/services/check.ts counted offenses per severity in three steps: an Object.prototype.hasOwnProperty.call probe, an explicit = 0 initialization, and a !-asserted increment. The non-null assertion was only safe because of the preceding initialization, so the reader has to follow all three lines to convince themselves the counter is sound.

WHAT is this pull request doing?

Collapse the accumulator update into a single nullish-coalescing expression, so the "start at zero, then add one" intent reads directly and the non-null assertion is no longer needed:

acc[offense.severity] = (acc[offense.severity] ?? 0) + 1

This matches the counting idiom already used elsewhere in the repo (for example packages/plugin-did-you-mean/src/index.ts).

Behavior is unchanged. The accumulator starts as {} and only ever stores numbers, so no key is ever explicitly set to undefined — the ?? 0 branch is taken in exactly the cases where the hasOwnProperty probe previously returned false.

How to manually test your changes?

pnpm shopify theme check

Run against a theme containing errors, warnings, and info-level offenses, and confirm the summary counts are unchanged. Also check the JSON shape:

pnpm shopify theme check --json

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

[Suleimanlatrsh]

Tested locally, including repeated severities and the text/JSON output. Looks good 👍