Fix console error caused by missing mount element

📖 What does this MR do?

Fixes console error on "subgroup" -> "Settings" -> "General" page. The Allow access to the following IP addresses and Restrict membership by email domain fields are only shown on the root groups and are not shown on subgroups. There is currently no mount element check in app/assets/javascripts/groups/settings/access_restriction_field/index.js and therefore console errors are shown on subgroups because the mount element does not exist. This MR adds a check to ensure the mount element exists before trying to mount the Vue app.

💻 Local testing

1. Create a group
2. Create a subgroup within that group
3. Open your dev tools
4. Navigate to "subgroup" -> "Settings" -> "General"

📷 Screenshots

Fields that were causing issues for context:

Before	After

🚦 Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is not a user facing change. The page still works even though there is currently a console error.
• Documentation (if required)
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• Style guides
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team