fix(29226): fix error for undefined unitInput for sending NFT (#29386)

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## **Description**
When switching to another non-ERC721 NFT in send flow, it is possible
that the input is undefined when rendered. As we didn't need to use
`scrollTo` for this input (introduced
[here](https://github.com/MetaMask/metamask-extension/pull/25307/files#diff-5b53d7ec060521a384c4dff7468398b01e302ac17511f243b79a5dd1d1cb69d3))
, putting a conditional running is sufficient.

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## **Related issues**

Fixes: #29226

## **Manual testing steps**

1. Go to NFT
2. Pick a non ERC721 NFT to send
3. Should not see errors in console when in the sending form

## **Screenshots/Recordings**

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### **Before**


https://github.com/user-attachments/assets/98268028-fcb1-419e-8638-bb3cbc482b3c


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### **After**


https://github.com/user-attachments/assets/830f40c3-0601-4eab-971f-eb69f253c084

<!-- [screenshots/recordings] -->

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ui/components/ui/unit-input/unit-input.component.js

@@ -95,7 +95,7 @@ export default class UnitInput extends PureComponent {
     }
 
     this.props.onBlur && this.props.onBlur(value);
-    this.unitInput.scrollTo && this.unitInput.scrollTo(0, 0);
+    this.unitInput?.scrollTo?.(0, 0);
   };
 
   handleChange = (event) => {