[skip ci] UIManagerJSInterface: Fix getConstantsForViewManager types

Summary:
If Flow can be trusted, getConstantsForViewManager always gets called with a non-null string:
1. The only call-site to getConstantsForViewManager is getViewManagerConfig: [PaperUIManager.js](https://github.com/facebook/react-native/blob/822bf52c29729d25b2bfb31655cf773609a9283d/packages/react-native/Libraries/ReactNative/PaperUIManager.js#L36-L80)
2. And getViewManagerConfig always passes in a non-null string.

So, let's just make the native argument type a non-nullable string.

Thoughts?

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D52628937

packages/react-native/Libraries/ReactNative/BridgelessUIManager.js

@@ -41,7 +41,7 @@ const getUIManagerConstantsCached = (function () {
   };
 })();
 
-const getConstantsForViewManager: ?(viewManagerName: string) => Object =
+const getConstantsForViewManager: ?(viewManagerName: string) => ?Object =
   global.RN$LegacyInterop_UIManager_getConstantsForViewManager;
 
 const getDefaultEventTypes: ?() => Object =
@@ -157,7 +157,7 @@ const UIManagerJSUnusedAPIs = {
 
 const UIManagerJSPlatformAPIs = Platform.select({
   android: {
-    getConstantsForViewManager: (viewManagerName: string): Object => {
+    getConstantsForViewManager: (viewManagerName: string): ?Object => {
       if (getConstantsForViewManager) {
         return getConstantsForViewManager(viewManagerName);
       }

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java

@@ -247,9 +247,8 @@ public static Map<String, Object> createConstants(
   }
 
   @ReactMethod(isBlockingSynchronousMethod = true)
-  public @Nullable WritableMap getConstantsForViewManager(@Nullable String viewManagerName) {
-    ViewManager targetView =
-        viewManagerName != null ? mUIImplementation.resolveViewManager(viewManagerName) : null;
+  public @Nullable WritableMap getConstantsForViewManager(String viewManagerName) {
+    ViewManager targetView = mUIImplementation.resolveViewManager(viewManagerName);
     if (targetView == null) {
       return null;
     }

packages/react-native/src/private/specs/modules/NativeUIManager.js

@@ -97,7 +97,7 @@ export interface Spec extends TurboModule {
   ) => void;
 
   // Android only
-  +getConstantsForViewManager?: (viewManagerName: string) => Object;
+  +getConstantsForViewManager?: (viewManagerName: string) => ?Object;
   +getDefaultEventTypes?: () => Array<string>;
   +setLayoutAnimationEnabledExperimental?: (enabled: boolean) => void;
   +sendAccessibilityEvent?: (reactTag: ?number, eventType: number) => void;