fix: retain array dtype in filter operations and avoid first() on empty results

[shayann07]

Fixes #221

filter, filterIndexed, filterMultiIndexed, filterNot, distinctBy, and partition in IteratingNDArray.kt were inferring the return array's dtype by calling DataType.of(list.first()). When the filtering predicate matched zero elements, list.first() threw NoSuchElementException instead of returning an empty array of the parent array's type.

Additionally, in ndarrayCommon (constructors.kt), val dtypeFromFirst = DataType.of(elements.first()) was evaluated eagerly even when an explicit dtype was supplied, causing crashes on empty collections.

This PR uses the parent array's this.dtype directly in these operations and lazily resolves dtype in ndarrayCommon. Added regression tests for empty filtering across all operations.

[shayann07]

Closing for now so the approach can be discussed in #221 first.