fix: Updated default config to accept undefined as default value (#2917)

Author: svetlanabrennan

lib/config/index.js

@@ -1042,7 +1042,7 @@ Config.prototype._fromPassed = function _fromPassed(external, internal, arbitrar
 
   Object.keys(external).forEach(function overwrite(key) {
     // if it's not in the defaults, it doesn't exist
-    if (!arbitrary && internal[key] === undefined) {
+    if (!arbitrary && !(key in internal)) {
       return
     }
 

test/unit/config/config-defaults.test.js

@@ -10,6 +10,7 @@ const assert = require('node:assert')
 const path = require('path')
 
 const Config = require('../../../lib/config')
+const proxyquire = require('proxyquire')
 
 test('with default properties', async (t) => {
   let configuration = null
@@ -316,3 +317,35 @@ test('with default properties', async (t) => {
     assert.equal(configuration.instrumentation.npmlog.enabled, true)
   })
 })
+
+test('with undefined as default', async (t) => {
+  const mockConfig = {
+    fake_key: {
+      another_layer: {
+        fake_nested_key: {
+          default: undefined
+        }
+      }
+    }
+  }
+
+  const defaults = require('../../../lib/config/default')
+  const orig = defaults.definition
+  defaults.definition = function stub() {
+    const configDefaults = orig.apply(this, arguments)
+    return { ...configDefaults, ...mockConfig }
+  }
+  const Config = proxyquire('../../../lib/config', {
+    './default': defaults
+  })
+
+  const configuration = Config.initialize({
+    fake_key: {
+      another_layer: {
+        fake_nested_key: 'fake-value'
+      }
+    }
+  })
+
+  assert.equal(configuration.fake_key.another_layer.fake_nested_key, 'fake-value')
+})