fix(vue): back button handler only fires in routing context

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue URL: N/A

While reviewing ionic-team/ionic-docs#2576, I noticed it was possible for ion-back-button to be used outside of a routing context. Clicking the back button causes the following error to be thrown:

Uncaught TypeError: Cannot read properties of undefined (reading 'handleNavigateBack')
    at onClick (@ionic_vue.js?v=fccd7e47:21998:15)
    at callWithErrorHandling (chunk-DPG7LVJ7.js?v=89dd46b5:1369:18)
    at callWithAsyncErrorHandling (chunk-DPG7LVJ7.js?v=89dd46b5:1377:17)
    at proxyCustomElement.color.invoker (chunk-DPG7LVJ7.js?v=89dd46b5:7512:7)

This error is happening because the onClick handler assumes the navManager injection is always defined.

What is the new behavior?

• Check to make sure navManager is defined before calling handleNavigateBack.

Does this introduce a breaking change?

• Yes
• No

Other information