Fix a server crash problem from pg_get_database_ddl

Hi,

While doing some testing, I hit a server crash:
```
2026-04-15 11:30:17.377 CST [98179] LOG: client backend (PID 41260) was terminated by signal 11: Segmentation fault: 11
2026-04-15 11:30:17.377 CST [98179] DETAIL: Failed process was running: SELECT * FROM pg_get_database_ddl('db1'::regdatabase);
2026-04-15 11:30:17.377 CST [98179] LOG: terminating any other active server processes
2026-04-15 11:30:17.380 CST [44361] FATAL: the database system is in recovery mode
```

After debugging it, I found that the crash happened because I had mistakenly deleted the tablespace entry directly from pg_tablespace, and pg_get_database_ddl_internal() calls get_tablespace_name() without checking whether the return value is NULL.

So this doesn't seem like a bug a normal user could hit. It is more like a superuser-only mistake that creates an invalid catalog state. I think that even in such an edge case, we should raise a proper error instead of crashing the backend.

BTW, I have verified that in this case, ALTER DATABASE ... SET TABLESPACE can move the database to a valid tablespace and recover from the issue.

This patch fixes that by checking for a NULL result and throwing an error.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/