Prevent NullPointerException exceptions in SimpleCache initialization

This is a temporary fix to prevent NullPointerException exceptions.
Though writing to cache will still fail if the cache folder isn't a real
folder.

There are a few thing we can try:
* The listing might be failing because the cache folder is just created.
We can wait and try again.
* If the cache folder is a regular file, we can try deleting and creating
a folder with the same name.

PiperOrigin-RevId: 234121925

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -582,6 +582,7 @@ public final class SimpleCache implements Cache {
    * @throws IOException If there is an error loading or generating the UID.
    */
   private static long loadUid(File directory, File[] files) throws IOException {
+    if (files != null) {
       for (File file : files) {
         String fileName = file.getName();
         if (fileName.endsWith(UID_FILE_SUFFIX)) {
@@ -594,6 +595,7 @@ public final class SimpleCache implements Cache {
           }
         }
       }
+    }
     return createUid(directory);
   }