[#9122] [Improvement]: Prevent potential null pointer error in MetadataObjectStatisticsOperations.java

[Pikady]

What changes were proposed in this pull request?

• Add argument validation in MetadataObjectStatisticsOperations#dropStatistics to throw IllegalArgumentException when the list is null or empty.
• Extend unit tests to assert null/empty list behavior.

Why are the changes needed?

• Prevents potential NullPointerException and provides clearer error feedback to callers.

Fix: #9122

Does this PR introduce any user-facing change?

• dropStatistics(null | empty) now throws IllegalArgumentException (null input previously could cause NPE).

How was this patch tested?

• Unit tests added for null and empty list inputs.