Fix NullPointerException when trying to add the vector's class name to the message for an UnsupportedOperationException

This test more closely follows the reproduction steps described in issue apache#10275

…o issue-10275

…eaderTest.java

Co-authored-by: Eduard Tudenhoefner <etudenhoefner@gmail.com>

…eaderTest.java

Co-authored-by: Eduard Tudenhoefner <etudenhoefner@gmail.com>

This solution hacks in a VectorHolder instance built specifically for the missing column. Implementing this hack allowed me to explore what would be needed to support vectorized reading of null columns

Update unit test to write a row after the schema has been altered. The test will then verify that all rows written both before and after the schema change can be correctly read.

Adding a second row was creating test complexity. The order in which the two rows are read asynchronously was creating randomness thus making it hard to predict the expected values. I'm not sure adding a second row of data was really adding any benefit anyway.

Those two test helper methods are highly tuned for a specific schema, a schema that does not exist in this test.

These unit tests, particularly `testIsDummy1` and `testIsDummy2`, exposed a bug in the code where the `isDummy` method no longer returned the expected value.

…eached

…Holder.java

…75-alt3