[tines] correct next_page value in system test rule config

[brijesh-elastic]

Proposed commit message

tines: correct `next_page` value in system test rule config to prevent updating
the fleet health status to degraded

Previously, the `next_page` value was incorrectly set to the string "null", so during system testing
it attempted to perform a new request with that URL (with the value `null`), resulting in an error.

Actually, it should be `"next_page": null` as mentioned in the (documentation)[1].

[1] https://www.tines.com/api/welcome/#the-meta-object

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

How to test this PR locally

• Clone integrations repo.
• Install elastic package locally.
• Start elastic stack using elastic-package.
• Move to integrations/packages/tines directory.
• Run the following command to run tests.

elastic-package test

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

Package tines 👍(1) 💚(0) 💔(1)

Expand to view

Data stream	Previous EPS	New EPS	Diff (%)	Result
time_saved	45454.55	10752.69	-34701.86 (-76.34%)	💔

To see the full report comment with /test benchmark fullreport

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 03391ec

cc @brijesh-elastic

[chrisberkhout]

Needs a changelog entry and version bump.

Change looks good.

[brijesh-elastic]

Needs a changelog entry and version bump.

We're only updating config.yml for the system test to pass. There are no user-facing changes. Do we still need to bump the version?