merge: json output doesn't include `dryRun` field when doing a noop merge

**Describe the bug** ```shell $ kart merge --no-editor --ff --fail-on-conflict --into=refs/heads/main 3972f9fbe477575ec1169070ab849bf261b82fb0 --output-format=json ``` This produces `"dryRun": true` when doing a normal merge, if merging into a branch that already has the given commit. However, when doing a noop merge (merging a commit that's already an ancestor of the branch), `dryRun` is not present **Version Info ** ``` $ kart --version Kart v0.16.2.dev0+ci.6460.gite9b7a55a, Copyright (c) Kart Contributors » GDAL v3.8.5; PROJ v9.4.0; PDAL v2.7.2 » PyGit2 v1.12.1; Libgit2 v1.6.4; Git v2.47.1; Git LFS v3.3.0 » SQLAlchemy v1.4.45; pysqlite3 v2.6.0/v3.45.3; SpatiaLite v5.1.0; Libpq v16.0.2 Executed via helper, SID=1 PID=647 ```