[BUG] Empty PPL query returns 500 NullPointerException instead of 400 #5867

What is the bug?
POST _plugins/_ppl with {"query": ""} returns 500:
Cannot invoke "org.opensearch.sql.ast.statement.Statement.accept(...)" because "plan" is null.

How can one reproduce the bug?
On 3.8.0: POST _plugins/_ppl {"query": ""}.

What is the expected behavior?
400 with a syntax error (e.g. "query is empty").

Analysis (code reading)
root: pplStatement? EOF accepts empty input, AstStatementBuilder returns null, and anonymizeStatement(null) throws:
https://github.com/opensearch-project/sql/blob/3.8.0.0/ppl/src/main/java/org/opensearch/sql/ppl/PPLService.java#L93-L114
https://github.com/opensearch-project/sql/blob/3.8.0.0/ppl/src/main/java/org/opensearch/sql/ppl/utils/PPLQueryDataAnonymizer.java#L152-L153
NPE is not classified as a client error, so it maps to 500. main still calls anonymizeStatement(statement) without a null check (I did not verify whether empty input is rejected earlier on main).

What is your host/environment? OpenSearch 3.8.0, SQL plugin 3.8.0.0
Related: opensearch-project/OpenSearch-Dashboards#12940