fix: consider packages key existence in pnpm-workspace.yaml

[sushichan044]

This PR allows bumpp to work properly in repositories where pnpm-workspace.yaml exists but not in a mono-repo configuration.

Description

In the latest version of pnpm, settings that were previously written under pnpm in package.json are now written in pnpm-workspace.yaml, as shown below.

onlyBuiltDependencies:
  - esbuild

patchedDependencies:
  bumpp: patches/bumpp.patch

Therefore, even if pnpm-workspace.yaml exists, it may not have a packages key.
So we must consider the case where the packages key is missing

Linked Issues

Additional context