fix: fix no-undefined-server-variable crashing on null in server list (#2142)

Author: RomanHotsiy

.changeset/good-vans-sell.md

@@ -0,0 +1,5 @@
+---
+"@redocly/cli": patch
+---
+
+Fixed `no-undefined-server-variable` crash when encountering `null` values in the server list.

packages/core/src/rules/oas3/__tests__/no-undefined-server-variables.test.ts

@@ -0,0 +1,26 @@
+import { outdent } from 'outdent';
+import { lintDocument } from '../../../lint.js';
+import { parseYamlToDocument, replaceSourceWithRef } from '../../../../__tests__/utils.js';
+import { BaseResolver } from '../../../resolve.js';
+import { createConfig } from '../../../config/index.js';
+
+describe('Oas3 no-undefined-server-variable', () => {
+  it('should not crash on null server', async () => {
+    const document = parseYamlToDocument(
+      outdent`
+          openapi: 3.0.0
+          servers:
+            - null
+        `,
+      'foobar.yaml'
+    );
+
+    const results = await lintDocument({
+      externalRefResolver: new BaseResolver(),
+      document,
+      config: await createConfig({ rules: { 'no-undefined-server-variable': 'error' } }),
+    });
+
+    expect(replaceSourceWithRef(results)).toHaveLength(0);
+  });
+});

packages/core/src/rules/oas3/no-undefined-server-variable.ts

@@ -3,7 +3,7 @@ import type { Oas3Rule } from '../../visitors.js';
 export const NoUndefinedServerVariable: Oas3Rule = () => {
   return {
     Server(server, { report, location }) {
-      if (!server.url) return;
+      if (!server?.url) return;
       const urlVariables = server.url.match(/{[^}]+}/g)?.map((e) => e.slice(1, e.length - 1)) || [];
       const definedVariables = (server?.variables && Object.keys(server.variables)) || [];