[Improvement] Possible NPE in SchemaOperations#createSchema

### What would you like to be improved?

SchemaOperations#createSchema logs request.getName() before entering its try/catch.
If request is null (for example, empty/missing POST body), this causes an immediate NullPointerException outside the error-handling path. Result: inconsistent 500 behavior.

### How should we improve?

Move all request field access behind a null-safe check inside the try path, and fail fast with a controlled validation error when request == null (for example, treat as bad request).
Also, make the catch path null-safe (don’t call request.getName() unless request is non-null).
Add/keep a unit test for null request input to ensure the endpoint never throws uncaught exceptions.

Here's a unit test to help:
```
@Test
public void testCreateSchemaWithNullRequestShouldNotThrow() {
SchemaOperations operations = new SchemaOperations(dispatcher);
Assertions.assertDoesNotThrow(() -> operations.createSchema(metalake, catalog, null));
}
```