fix: prevent exception when imageName is null (#20120)

Summary:
When an image source is parsed via `RCTImageFromLocalAssetURL` there seem to be certain situations in which `imageName` is empty from `RCTBundlePathForURL`. Any call to `UIImage imageNamed` with a an empty parameter will throw an exception:

```
CUICatalog: Invalid asset name supplied: '(null)'
```

In my case, the asset URL was pointing to an image in the application sandbox rather than the `NSBundle`. In this case `UIImage imageNamed` was throwing before the call to `NSData dataWithContentsOfURL` below could correctly resolve the image.

This change simply skips the call to `UIImage imageNamed` if no `imageName` value is set.
Pull Request resolved: #20120

Differential Revision: D14163101

Pulled By: cpojer

fbshipit-source-id: ceec95c02bf21b739962ef5618947a5726ba0473

Author: Ignigena

Diff for: React/Base/RCTUtils.m

@@ -700,10 +700,12 @@ BOOL RCTIsLocalAssetURL(NSURL *__nullable imageURL)
   }
 
   UIImage *image = nil;
-  if (bundle) {
-    image = [UIImage imageNamed:imageName inBundle:bundle compatibleWithTraitCollection:nil];
-  } else {
-    image = [UIImage imageNamed:imageName];
+  if (imageName) {
+    if (bundle) {
+      image = [UIImage imageNamed:imageName inBundle:bundle compatibleWithTraitCollection:nil];
+    } else {
+      image = [UIImage imageNamed:imageName];
+    }
   }
 
   if (!image) {