Parameters annotated with @Nullable fail with null in Builders

[fhnaumann]

Describe the bug
The builder methods in DataSinkBuilder and DataSourceBuilder have many methods annotated with @Nullable but they are checked to be non-null.

To Reproduce

new DataSourceBuilder(pathlingContext).parquet(null) // error, but annotated with nullable

Expected behavior
Many (if not all?) @Nullable annotations should be replaced with @Nonnull.

[johngrimes]

This is a little bit tricky, I'm not sure if I understand the right thing to do here.

If we annotation these with @Nonnull, are we assuming that nulls will not be passed? If I change it to @Nonnull, then I will no longer get a warning if I remove the null argument check.

Do I want the annotations to help me make sure that the code within these methods is correct, or to help make sure that code that is calling these methods is correct?

The other factor here is that these methods get called not just from Java/Scala code but also Python and R code, so we can't rely upon that code to be statically checked to make sure that it doesn't pass nulls to @Nonnull annotated parameters.

@piotrszul What do you think?