mergeConfig: Fix handling of partial config first argument

Summary:
Typically the `metro-config` `mergeConfig` API is called with a fully populated config as its first argument (often `getDefaultValues(__dirname)`), but it has in fact always supported merging a partial config (`InputConfigT`) into another.

This has a couple of bugs, however:
 1. When the base config lacks a top-level `resolver` or `tranformer` config, the call fails hard on accessing a prop of `undefined`.
 2. When neither config contains one of `cacheStores`, `transformer.babelTransformerPath`, `resolver.dependencyExtractor` or `resolver.hasteImplModulePath`, `mergeConfig` will cause that property to be set with a value of `undefined`, which could cause issues if that is subsequently spread over a config where they are populated.

```
 - **[Fix]** Fix `mergeConfig` handling of 2+ partial configs
```

Reviewed By: motiz88

Differential Revision: D83475279

fbshipit-source-id: 7e181d0ab0526cd090b321dd2822c4339b430c6f

Author: robhogan

packages/metro-config/src/__tests__/mergeConfig-test.js

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+import {mergeConfig} from '../loadConfig';
+
+describe('mergeConfig', () => {
+  test('can merge empty configs', () => {
+    expect(mergeConfig({}, {})).toStrictEqual({
+      resolver: {},
+      serializer: {},
+      server: {},
+      symbolicator: {},
+      transformer: {},
+      watcher: {
+        healthCheck: {},
+        unstable_autoSaveCache: {},
+        watchman: {},
+      },
+    });
+  });
+});

packages/metro-config/src/loadConfig.js

@@ -110,54 +110,53 @@ function mergeConfigObjects<T: InputConfigT>(
   return {
     ...base,
     ...overrides,
-
-    cacheStores:
-      overrides.cacheStores != null
-        ? typeof overrides.cacheStores === 'function'
-          ? overrides.cacheStores(MetroCache)
-          : overrides.cacheStores
-        : base.cacheStores,
+    ...(overrides.cacheStores != null
+      ? {
+          cacheStores:
+            typeof overrides.cacheStores === 'function'
+              ? overrides.cacheStores(MetroCache)
+              : overrides.cacheStores,
+        }
+      : null),
 
     resolver: {
       ...base.resolver,
+      ...overrides.resolver,
       // $FlowFixMe[exponential-spread]
-      ...(overrides.resolver || {}),
-      dependencyExtractor:
-        overrides.resolver && overrides.resolver.dependencyExtractor != null
-          ? resolve(overrides.resolver.dependencyExtractor)
-          : // $FlowFixMe[incompatible-use]
-            base.resolver.dependencyExtractor,
-      hasteImplModulePath:
-        overrides.resolver && overrides.resolver.hasteImplModulePath != null
-          ? resolve(overrides.resolver.hasteImplModulePath)
-          : // $FlowFixMe[incompatible-use]
-            base.resolver.hasteImplModulePath,
+      ...(overrides.resolver?.dependencyExtractor != null
+        ? {dependencyExtractor: resolve(overrides.resolver.dependencyExtractor)}
+        : null),
+      ...(overrides.resolver?.hasteImplModulePath != null
+        ? {hasteImplModulePath: resolve(overrides.resolver.hasteImplModulePath)}
+        : null),
     },
     serializer: {
       ...base.serializer,
       // $FlowFixMe[exponential-spread]
-      ...(overrides.serializer || {}),
+      ...overrides.serializer,
     },
     transformer: {
       ...base.transformer,
       // $FlowFixMe[exponential-spread]
-      ...(overrides.transformer || {}),
-      babelTransformerPath:
-        overrides.transformer &&
-        overrides.transformer.babelTransformerPath != null
-          ? resolve(overrides.transformer.babelTransformerPath)
-          : // $FlowFixMe[incompatible-use]
-            base.transformer.babelTransformerPath,
+      ...overrides.transformer,
+      // $FlowFixMe[exponential-spread]
+      ...(overrides.transformer?.babelTransformerPath != null
+        ? {
+            babelTransformerPath: resolve(
+              overrides.transformer.babelTransformerPath,
+            ),
+          }
+        : null),
     },
     server: {
       ...base.server,
       // $FlowFixMe[exponential-spread]
-      ...(overrides.server || {}),
+      ...overrides.server,
     },
     symbolicator: {
       ...base.symbolicator,
       // $FlowFixMe[exponential-spread]
-      ...(overrides.symbolicator || {}),
+      ...overrides.symbolicator,
     },
     watcher: {
       ...base.watcher,