Fix Jira importer immutable cache update

What does this MR do?

This MR fixes an error raised in #255929 (closed) and due to !43318 (merged).

On staging, the following error is printed in the browser console and could be due to the push invoking Vue's reactivity.

 TypeError: a is undefined
    jB52 VueJS
    data cache_update.js:25
    produce immer.esm.js:1
    $ cache_update.js:23
    update jira_import_form.vue:202
    markMutationResult bundle.esm.js:1870
    Y bundle.esm.js:802
instrument.js:108:45

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• 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