[Spark] Tidy ArrowFieldWriter: generic vector field and a single place that advances count #14

ArrowFieldWriter (PR #7) has two small structural issues worth tidying. Neither is a bug today.

1. Each subclass copies the base vector field

17 subclasses keep a second, typed copy of the base vector (bits, bytes, shorts, ...), each with a constructor that only sets it.

Fix: make the base class generic, ArrowFieldWriter<V extends FieldVector> with a final V vector, and drop the copies.

2. count is advanced in two places

write() increments count, but StructWriter.setNull also increments each child's count by hand. A future writer that forgets this misaligns the struct's children with no error.

Fix: a writeNull() helper that calls setNull() and then count++, used for nulled struct children.

Where

• ArrowFieldWriter.java. The manual increment is at L343.