[BUG] NPE risk in AuronUniffleShuffleReader

[slfan1989]

Describe the bug

In AuronUniffleShuffleReader, there are potential NullPointerException risks when handling partitions:

• partitionToExpectBlocks.get(partition) may return null, but the code directly calls .isEmpty() without null check
• partitionToShuffleServers.get(partition) may return null, but the code uses it directly without validationThis can cause the shuffle reader to crash when reading partitions that have missing or incomplete metadata.
  This can cause the shuffle reader to crash when reading partitions that have missing or incomplete metadata.