Which issue does this PR close?

Closes #5712

Rationale for this change

We would like to create random arrays for testing and leveraging data_gen module but it current breaks with our schema of:

List not-nullable
    Struct nullable
        field_a nullable
        field_b nullable

More details are provided on issue #5712

What changes are included in this PR?

The changes are isolated to the create_random_array and create_random_list_array functions. Previously the code would assume if the field is not null, set null_density to zero. However that zero is then passed down into nested fields' create_random_array calls, thus resulting in no nulls in those arrays. This PR preserves the original null_density to pass into nested create_random_array calls.

Are there any user-facing changes?

There is a behavior change with array generation with schemas that have nested nullable fields under non-nullable fields.