fix(iOS): clear default NukeUI placeholder background

NukeUI's LazyImageView creates a default placeholderView with
secondarySystemBackground color (#1c1c1e in dark mode).

This prevents users from customizing the background color via
React Native styles, as the placeholder covers the entire view.

Setting placeholderView to nil in init allows the parent view's
backgroundColor (set via RN style prop) to be visible.

Fixes #405