fix(config/parse): merge extends iff additionalConfigFile has extends (renovatebot#37717)

Author: RahulGautamSingh

lib/workers/global/config/parse/index.spec.ts

@@ -337,5 +337,16 @@ describe('workers/global/config/parse/index', () => {
         'customManagers:azurePipelinesVersions',
       ]);
     });
+
+    it('adds extends from fileConfig only', async () => {
+      fileConfigParser.getConfig.mockResolvedValueOnce({
+        extends: [':pinDigests'],
+      });
+      const parsedConfig = await configParser.parseConfigs(
+        defaultEnv,
+        defaultArgv,
+      );
+      expect(parsedConfig.extends).toMatchObject([':pinDigests']);
+    });
   });
 });

lib/workers/global/config/parse/index.ts

@@ -50,7 +50,10 @@ export async function parseConfigs(
 
   let config: AllConfig = mergeChildConfig(fileConfig, additionalFileConfig);
   // merge extends from file config and additional file config
-  if (is.nonEmptyArray(fileConfig.extends)) {
+  if (
+    is.nonEmptyArray(fileConfig.extends) &&
+    is.nonEmptyArray(additionalFileConfig.extends)
+  ) {
     config.extends = [...fileConfig.extends, ...(config.extends ?? [])];
   }
   config = mergeChildConfig(config, envConfig);