Fix NPE in PostgreSQL plugin when executing statements prepared with generated keys

[carteruu]

Fix the NullPointerException in the PostgreSQL plugin for statements prepared with generated keys (apache/skywalking#14138)

• Add a unit test to verify that the fix works.
• Explain briefly why the bug exists and how to fix it.

PgConnection#prepareStatement(String, String[]) is intercepted by JDBCPrepareStatementWithStringArrayInterceptor, which wraps the result in SWPreparedStatement but does not set the dynamic field of the real PgPreparedStatement. pgjdbc 42.x constructs that statement directly instead of going through the intercepted 4-argument prepareStatement, so its dynamic field stays null. prepareStatement(String, int) with Statement.RETURN_GENERATED_KEYS delegates to this method, and Hibernate and MyBatis use it for generated keys.

PgPreparedStatement is still enhanced, so when SWPreparedStatement calls the real executeUpdate (and the nested executeWithFlags), PreparedStatementExecuteMethodsInterceptor dereferences the null dynamic field. The agent logs 4 NPEs for every such insert, while the SQL span is still recorded by SWPreparedStatement.

This PR skips the interceptor when the dynamic field or its ConnectionInfo is null, which is the same guard the MySQL plugin's PreparedStatementExecuteMethodsInterceptor already has. Tracing of this path is unchanged. The new unit test fails with the NPE from the issue before this change.

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes [Bug] PostgreSQL plugin throws NPE when executing statements prepared with RETURN_GENERATED_KEYS skywalking#14138.
• Update the CHANGES log.