Avoid breaking on fieldConfig without defaults field (#36666)

This would result in a `Dashboard init failed` error when migrating
dashboards with a folded panel that has a `fieldConfig` but has not
defined `fieldConfig.defaults`.

public/app/features/dashboard/state/DashboardMigrator.test.ts

@@ -1382,6 +1382,44 @@ describe('DashboardModel', () => {
       `);
     });
   });
+
+  describe('when migrating folded panel without fieldConfig.defaults', () => {
+    let model: DashboardModel;
+
+    beforeEach(() => {
+      model = new DashboardModel({
+        schemaVersion: 29,
+        panels: [
+          {
+            id: 1,
+            type: 'timeseries',
+            panels: [
+              {
+                id: 2,
+                fieldConfig: {
+                  overrides: [
+                    {
+                      matcher: { id: 'byName', options: 'D-series' },
+                      properties: [
+                        {
+                          id: 'displayName',
+                          value: 'foobar',
+                        },
+                      ],
+                    },
+                  ],
+                },
+              },
+            ],
+          },
+        ],
+      });
+    });
+
+    it('should ignore fieldConfig.defaults', () => {
+      expect(model.panels[0].panels[0].fieldConfig.defaults).toEqual(undefined);
+    });
+  });
 });
 
 function createRow(options: any, panelDescriptions: any[]) {

public/app/features/dashboard/state/DashboardMigrator.ts

@@ -955,7 +955,12 @@ function upgradeValueMappingsForPanel(panel: PanelModel) {
     return panel;
   }
 
-  fieldConfig.defaults.mappings = upgradeValueMappings(fieldConfig.defaults.mappings, fieldConfig.defaults.thresholds);
+  if (fieldConfig.defaults) {
+    fieldConfig.defaults.mappings = upgradeValueMappings(
+      fieldConfig.defaults.mappings,
+      fieldConfig.defaults.thresholds
+    );
+  }
 
   // Protect against no overrides
   if (Array.isArray(fieldConfig.overrides)) {