sql: regression: invalid assignment cast: unknown -> int[]

[rafiss]

Describe the problem

Previously, before assignment casts were added, the following PGJDBC test would pass.

https://github.com/pgjdbc/pgjdbc/blob/207ce36ec2c5cce30c454afa3e51f39bbe5bd26b/pgjdbc/src/test/java/org/postgresql/test/jdbc2/ArrayTest.java#L340

  @Test
  public void testSetNullArrays() throws SQLException {
    PreparedStatement pstmt = conn.prepareStatement("INSERT INTO arrtest VALUES (?,?,?)");

    final PGConnection arraySupport = conn.unwrap(PGConnection.class);

    pstmt.setArray(1, arraySupport.createArrayOf("int4", null));
    pstmt.setObject(2, conn.createArrayOf("float8", null));
    pstmt.setObject(3, arraySupport.createArrayOf("varchar", null));

    pstmt.executeUpdate();
    pstmt.close();

    Statement stmt = conn.createStatement();
    ResultSet rs = stmt.executeQuery("SELECT intarr, decarr, strarr FROM arrtest");
    Assert.assertTrue(rs.next());

    Array arr = rs.getArray(1);
    Assert.assertNull(arr);

    arr = rs.getArray(2);
    Assert.assertNull(arr);

    arr = rs.getArray(3);
    Assert.assertNull(arr);

    rs.close();
  }

Now it fails with

�[0;31;1mFAILURE�[0m   0.1sec, org.postgresql.test.jdbc2.�[0;1mArrayTest�[0m > �[0;1mtestSetNullArrays�[0m[binary = REGULAR]
    org.postgresql.util.PSQLException: ERROR: invalid assignment cast: unknown -> int[]
        at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2553)
        at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2285)
        at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:323)
        at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:481)
        at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:401)
        at org.postgresql.jdbc.PgPreparedStatement.executeWithFlags(PgPreparedStatement.java:164)
        at org.postgresql.jdbc.PgPreparedStatement.executeUpdate(PgPreparedStatement.java:130)
        �[0;1mat org.postgresql.test.jdbc2.ArrayTest.testSetNullArrays(ArrayTest.java:349)

To Reproduce

Here's a repro that you can do from the CLI. This used to work with CRDB and still works with PG, but now it fails on master:

create table arrtest (intarr int8[]);
prepare q as INSERT INTO arrtest VALUES($1::int4[]);
execute q(null);

Now it results in

ERROR: invalid assignment cast: unknown -> int[]
SQLSTATE: 42846

Additional context
Causing #71564 to fail

cc @mgartner

[rafiss]

I added a simpler repro to the original issue

[rafiss]

Fixed by #72575