cli: allow explicit null pluginId in package metadata

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/cli-node/src/monorepo/PackageGraph.ts

@@ -50,7 +50,7 @@ export interface BackstagePackageJson {
     /**
      * The ID of the plugin if this is a plugin package. Must always be set for plugin and module packages, and may be set for library packages.
      */
-    pluginId?: string;
+    pluginId?: string | null;
 
     /**
      * The parent plugin package of a module. Must always and only be set for module packages.

packages/cli/src/commands/repo/fix.ts

@@ -277,7 +277,7 @@ export function fixPluginId(pkg: FixablePackage) {
 
   const currentId = pkg.packageJson.backstage?.pluginId;
   if (currentId !== undefined) {
-    if (typeof currentId !== 'string') {
+    if (typeof currentId !== 'string' && currentId !== null) {
       throw new Error(
         `Invalid 'backstage.pluginId' field in "${pkg.packageJson.name}", must be a string`,
       );