NullPointerException when using VectorizedArrowReader to read a null column

### Apache Iceberg version

1.5.1 (latest release)

### Query engine

Other

### Please describe the bug 🐞

I am writing a compatibility layer for Teradata so that it can access Iceberg tables stored in AWS S3. I am experiencing what at first glance appears to be a bug in Iceberg, but I'd like to get the opinion of the experts here. To be clear I am using Apache Iceberg 1.5.1 and Apache Arrow 15.0.0.

The problem is I am getting a NullPointerException thrown from [GenericArrowVectorFactory.java line 224](https://github.com/apache/iceberg/blob/cbb853073e681b4075d7c8707610dceecbee3a82/arrow/src/main/java/org/apache/iceberg/arrow/vectorized/GenericArrowVectorAccessorFactory.java#L224). The NPE is thrown on line 224 because `vector` is null.
```
throw new UnsupportedOperationException("Unsupported vector: " + vector.getClass());
```
How do I get to this point? Here's the minimal test case:

**Prerequisite:**
```
create table otf920ath (
a INT NOT NULL,
b string(10),
c decimal(12, 3)
)
LOCATION 's3://*******************'
TBLPROPERTIES ('table_type' = 'ICEBERG');

INSERT INTO otf920ath values (1, 'san diego', 1024.025);

ALTER TABLE otf920ath
ADD COLUMNS (a1 int);
```
**repro:**
```
select * from otf920ath;
```
The above SQL select statement works in AWS Athena, but fails in my code. My code is using an instance of `org.apache.iceberg.arrow.vectorized.ArrowReader$VectorizedCombinedScanIterator`

The cause, as I see it, is that the one row in the table contains only three columns worth of data, but the current table schema defines four columns. Because of this difference in schemas Iceberg creates the following four readers, once for each column respectively:
`VecorizedArrowReader` corresponding to column `a`
`VecorizedArrowReader` corresponding to column `b`
`VecorizedArrowReader` corresponding to column `c`
`VecorizedArrowReader$NullVectorReader` corresponding to column `a1`

Naturally the `VecorizedArrowReader$NullVectorReader` instance contains a `null` value for the vector. This instance is assigned at [VectorizedReaderBuilder.java line 100](https://github.com/apache/iceberg/blob/cbb853073e681b4075d7c8707610dceecbee3a82/arrow/src/main/java/org/apache/iceberg/arrow/vectorized/VectorizedReaderBuilder.java#L100).

Continuing down the code path Iceberg calls `GenericArrowVectorAccessorFactory.getPlainVectorAccessor`. This method checks to see whether `vector` is an instance of various *Vector types. Because `vector` has a value of `null` it is not an instance of any type. Thus this method ends up in its ultimate fallback case and tries to throw an exception:
```
throw new UnsupportedOperationException("Unsupported vector: " + vector.getClass());
```
The problem is that `vector` is `null` and this calling `vector.getClass()` throws a `NullPointerException`.

The stack trace is:
```
java.lang.NullPointerException
at org.apache.iceberg.arrow.vectorized.GenericArrowVectorAccessorFactory.getPlainVectorAccessor(GenericArrowVectorAccessorFactory.java:224)
at org.apache.iceberg.arrow.vectorized.GenericArrowVectorAccessorFactory.getVectorAccessor(GenericArrowVectorAccessorFactory.java:110)
at org.apache.iceberg.arrow.vectorized.ArrowVectorAccessors.getVectorAccessor(ArrowVectorAccessors.java:54)
at org.apache.iceberg.arrow.vectorized.ColumnVector.getVectorAccessor(ColumnVector.java:136)
at org.apache.iceberg.arrow.vectorized.ColumnVector.(ColumnVector.java:56)
at org.apache.iceberg.arrow.vectorized.ArrowBatchReader.read(ArrowBatchReader.java:54)
at org.apache.iceberg.arrow.vectorized.ArrowBatchReader.read(ArrowBatchReader.java:29)
at org.apache.iceberg.parquet.VectorizedParquetReader$FileIterator.next(VectorizedParquetReader.java:149)
at org.apache.iceberg.arrow.vectorized.ArrowReader$VectorizedCombinedScanIterator.next(ArrowReader.java:314)
at org.apache.iceberg.arrow.vectorized.ArrowReader$VectorizedCombinedScanIterator.next(ArrowReader.java:190)
```

**So my questions:**
1. Is it possible that this is a bug in Iceberg?
2. If so, is the fix simply to handle the `null` value for `vector` when building the message for the UnsupportedOperationException?
3. If not, is there some other code path or method arguments I should be using?

p.s. I asked this question in the Slack channel but didn't get any traction. https://apache-iceberg.slack.com/archives/C025PH0G1D4/p1714676216273989

Start with GenericArrowVectorAccessorFactory.getPlainVectorAccessor at line 224 and VectorizedReaderBuilder.java line 100, then follow the null-column path through ColumnVector and ArrowBatchReader. Reproduce the failure with the provided schema-evolution SQL and VectorizedCombinedScanIterator. Done means the null-column case follows the intended error or reader behavior without an unexpected NullPointerException.

Written by the indexing model from the issue text.