Fix #6029 Avoid undefined checksums after merge conflict resolution

.yarn/versions/4c645b68.yml

@@ -0,0 +1,34 @@
+releases:
+  "@yarnpkg/cli": patch
+  "@yarnpkg/plugin-essentials": patch
+
+declined:
+  - "@yarnpkg/core"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-npm"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-constraints"
+  - "@yarnpkg/plugin-dlx"
+  - "@yarnpkg/plugin-exec"
+  - "@yarnpkg/plugin-file"
+  - "@yarnpkg/plugin-git"
+  - "@yarnpkg/plugin-github"
+  - "@yarnpkg/plugin-http"
+  - "@yarnpkg/plugin-init"
+  - "@yarnpkg/plugin-link"
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/plugin-pack"
+  - "@yarnpkg/plugin-patch"
+  - "@yarnpkg/plugin-pnp"
+  - "@yarnpkg/plugin-pnpm"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/builder"
+  - "@yarnpkg/doctor"
+  - "@yarnpkg/extensions"
+  - "@yarnpkg/nm"
+  - "@yarnpkg/pnpify"
+  - "@yarnpkg/sdks"

packages/plugin-essentials/sources/commands/install.ts

@@ -455,7 +455,7 @@ async function autofixMergeConflicts(configuration: Configuration, immutable: bo
         continue;
 
       const checksum = variant[key].checksum;
-      if (typeof checksum === `string` && checksum.includes(`/`))
+      if (typeof checksum === `undefined` || checksum.includes(`/`))
         continue;
 
       variant[key].checksum = `${variant.__metadata.cacheKey}/${checksum}`;