check for undefined values on setPluginResolvedOn function

packages/server/lib/config.js

@@ -506,7 +506,7 @@ module.exports = {
   // {value: obj.val, from: "plugin"}
   setPluginResolvedOn (resolvedObj, obj) {
     return _.each(obj, (val, key) => {
-      if (_.isObject(val) && !_.isArray(val)) {
+      if (_.isObject(val) && !_.isArray(val) && resolvedObj[key]) {
         // recurse setting overrides
         // inside of this nested objected
         return this.setPluginResolvedOn(resolvedObj[key], val)

packages/server/test/unit/config_spec.js

@@ -1309,6 +1309,32 @@ describe('lib/config', () => {
         },
       })
     })
+
+    // https://github.com/cypress-io/cypress/issues/7959
+    it('resolves a single object', () => {
+      const cfg = {
+      }
+      const obj = {
+        foo: {
+          bar: {
+            baz: 42,
+          },
+        },
+      }
+
+      config.setPluginResolvedOn(cfg, obj)
+
+      expect(cfg).to.deep.eq({
+        foo: {
+          from: 'plugin',
+          value: {
+            bar: {
+              baz: 42,
+            },
+          },
+        },
+      })
+    })
   })
 
   context('_.defaultsDeep', () => {