fix(editor): connector target position NaN (#11606)

Close [BS-3086](https://linear.app/affine-design/issue/BS-3086/frame里套frame，连一下connector，拖两下，白板损坏)

### What Changes
- Fixed `bound.toRelative` may be return `NaN` when `bound.w === 0 || bound.h ===0`
- Remove type assertions from `connector-manager.ts` for more type safety

blocksuite/framework/std/src/gfx/viewport.ts

@@ -214,7 +214,12 @@ export class Viewport {
    * This property is used to calculate the scale of the editor.
    */
   get viewScale() {
-    if (!this._shell || this._cachedOffsetWidth === null) return 1;
+    if (
+      !this._shell ||
+      this._cachedOffsetWidth === null ||
+      this._cachedOffsetWidth === 0
+    )
+      return 1;
     return this.boundingClientRect.width / this._cachedOffsetWidth;
   }