fix(item-sliding): check for side attribute to avoid an undefined value (#29845)

Issue number: resolves #29499

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## What is the current behavior?
Using the bundled version of Ionic results in an error `Uncaught (in
promise) Error: "undefined" is not a valid value for [side]. Use "start"
or "end" instead.` with `<ion-item-sliding>`.

Reproduction 1: 
- https://stackblitz.com/edit/stackblitz-starters-mezoy6?file=index.html
- Press the Add (`+`) button several times

It is also reproducible in an Ionic Angular app when installed via npm.

Reproduction 2: 
-
https://stackblitz.com/edit/angular-exwgke?file=src%2Fapp%2Fexample.component.ts
- Press the `Add Items` button several times

## What is the new behavior?
Check for the side attribute to avoid `side` being `undefined`.

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

## Other information

Dev build: `8.6.1-dev.11749562115.1e681558`

Test PR: #30469

The tests must be merged separately, after this is released, in order to
use the fix from the CDN.

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Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>

Author: brandyscarney

core/src/components/item-sliding/item-sliding.tsx

@@ -263,7 +263,7 @@ export class ItemSliding implements ComponentInterface {
       // eslint-disable-next-line custom-rules/no-component-on-ready-method
       const option = (item as any).componentOnReady !== undefined ? await item.componentOnReady() : item;
 
-      const side = isEndSide(option.side) ? 'end' : 'start';
+      const side = isEndSide(option.side ?? option.getAttribute('side')) ? 'end' : 'start';
 
       if (side === 'start') {
         this.leftOptions = option;