fix: hide network picker back button when network is unselected (#29711)

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## **Description**

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Write a short description of the changes included in this pull request,
also include relevant motivation and context. Have in mind the following
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1. What is the reason for the change?
2. What is the improvement/solution?
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This hides the network picker's back button when there's no selected
network. Currently this causes the dest network picker to navigate to
the src asset picker

[![Open in GitHub
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## **Related issues**

Fixes: https://consensyssoftware.atlassian.net/browse/MMS-1852

## **Manual testing steps**

1. Load bridge page
2. Open dest asset picker
3. Verify that back button is not visible

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

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


https://github.com/user-attachments/assets/4bbde8e4-bdd5-4fa2-aeb5-248d433e25a2


### **After**

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


https://github.com/user-attachments/assets/731280a5-3faf-401c-9236-8e932a9f347e



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- [X] I've completed the PR template to the best of my ability
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Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

ui/components/multichain/asset-picker-amount/asset-picker-modal/__snapshots__/asset-picker-modal-network.test.tsx.snap

@@ -32,20 +32,6 @@ exports[`AssetPickerModalNetwork renders modal with no network list by default 1
           <header
             class="mm-box mm-header-base mm-modal-header mm-box--padding-right-4 mm-box--padding-bottom-4 mm-box--padding-left-4 mm-box--display-flex mm-box--justify-content-space-between"
           >
-            <div
-              class="mm-box"
-              style="min-width: 0px;"
-            >
-              <button
-                aria-label="Back"
-                class="mm-box mm-button-icon mm-button-icon--size-sm mm-box--display-inline-flex mm-box--justify-content-center mm-box--align-items-center mm-box--color-icon-default mm-box--background-color-transparent mm-box--rounded-lg"
-              >
-                <span
-                  class="mm-box mm-icon mm-icon--size-sm mm-box--display-inline-block mm-box--color-inherit"
-                  style="mask-image: url('./images/icons/arrow-left.svg');"
-                />
-              </button>
-            </div>
             <div
               class="mm-box mm-box--width-full"
             >
@@ -121,20 +107,6 @@ exports[`AssetPickerModalNetwork should not show selected network when network p
           <header
             class="mm-box mm-header-base mm-modal-header mm-box--padding-right-4 mm-box--padding-bottom-4 mm-box--padding-left-4 mm-box--display-flex mm-box--justify-content-space-between"
           >
-            <div
-              class="mm-box"
-              style="min-width: 0px;"
-            >
-              <button
-                aria-label="Back"
-                class="mm-box mm-button-icon mm-button-icon--size-sm mm-box--display-inline-flex mm-box--justify-content-center mm-box--align-items-center mm-box--color-icon-default mm-box--background-color-transparent mm-box--rounded-lg"
-              >
-                <span
-                  class="mm-box mm-icon mm-icon--size-sm mm-box--display-inline-block mm-box--color-inherit"
-                  style="mask-image: url('./images/icons/arrow-left.svg');"
-                />
-              </button>
-            </div>
             <div
               class="mm-box mm-box--width-full"
             >

ui/components/multichain/asset-picker-amount/asset-picker-modal/asset-picker-modal-network.tsx

@@ -160,7 +160,7 @@ export const AssetPickerModalNetwork = ({
       <ModalOverlay />
       <ModalContent modalDialogProps={{ padding: 0 }}>
         <ModalHeader
-          onBack={onBack}
+          onBack={network ? onBack : undefined}
           onClose={isMultiselectEnabled ? undefined : onClose}
           endAccessory={
             isMultiselectEnabled && selectedChainIds ? (