Rework has_cluster_privilege to error intentionally (#28034)

The `has_cluster_privilege` function errors through
`mz_unsafe.mz_error_if_null`, but it then uses the result in an `IS
NULL` test, which is able to be optimized away (as
`mz_unsafe.mz_error_if_null` cannot return `NULL`).

This PR changes the logic to test the condition explicitly and error if
the test fails, rather than rely on the optimizer not noticing that the
potentially erroring test can be pruned.

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src/sql/src/func.rs

@@ -3550,15 +3550,15 @@ pub static MZ_CATALOG_BUILTINS: Lazy<BTreeMap<&'static str, Func>> = Lazy::new(|
             params!(String, String, String) => sql_impl_func("has_cluster_privilege(mz_internal.mz_role_oid($1), $2, $3)") => Bool, oid::FUNC_HAS_CLUSTER_PRIVILEGE_TEXT_TEXT_TEXT_OID;
             params!(Oid, String, String) => sql_impl_func(&format!("
                 CASE
-                -- We need to validate the name and privileges to return a proper error before
-                -- anything else.
-                WHEN mz_unsafe.mz_error_if_null(
-                    (SELECT name FROM mz_clusters WHERE name = $2),
-                    'error cluster \"' || $2 || '\" does not exist'
-                ) IS NULL
-                OR NOT mz_internal.mz_validate_privileges($3)
+                -- We must first check $2 to avoid a potentially null error message (an error itself).
+                WHEN $2 IS NULL
+                THEN NULL
+                -- Validate the cluster name in order to return a proper error.
+                WHEN NOT EXISTS (SELECT name FROM mz_clusters WHERE name = $2)
+                THEN mz_unsafe.mz_error_if_null(NULL::boolean, 'error cluster \"' || $2 || '\" does not exist')
+                -- Validate the privileges and other arguments.
+                WHEN NOT mz_internal.mz_validate_privileges($3)
                 OR $1 IS NULL
-                OR $2 IS NULL
                 OR $3 IS NULL
                 OR $1 NOT IN (SELECT oid FROM mz_catalog.mz_roles)
                 THEN NULL