beta: Fix undefined variable reference

[anomiex]

Proposed changes:

$plugin wasn't defined when show-needed-updates.template.php was used from plugin-select.template.php. Define it as null.

Also remove an unnecessary isset(), by that point it is defined.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Visit the plugin select page. Check for warnings.

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