fix: prevent split-layout from overflowing parent flex container (#11462)

## Description

Add `min-height: 0` to the `:host` rule so that the `split-layout`, when
used as a flex item, can shrink below its content's natural height.
Without this, a grid inside a horizontal `split-layout` in a flex column
parent expands to full content height instead of scrolling internally.

Fixes #11362

## Type of change

- Bugfix

Author: DiegoCardoso

packages/split-layout/src/styles/vaadin-split-layout-base-styles.js

@@ -10,6 +10,7 @@ export const splitLayoutStyles = css`
   :host {
     display: flex;
     contain: layout;
+    min-height: 0;
   }
 
   :host([hidden]) {