Fix PolicyScopeFetcher when policy_scope is empty

What does this MR do and why?

This MR fixes a bug with Security::SecurityOrchestrationPolicies::PolicyScopeFetcher where if policy_scope is empty, it throws an error. This was introduced as a part of Expose policy_scope in policy graphql API (!142248 - merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Addresses #443862 (closed)

changed milestone to %16.10

added Category:Security Policy Management devopsgovern groupsecurity policies sectionsec typebug labels

assigned to @sashi_kumar

mentioned in issue #432513 (closed)

added backend label

mentioned in issue #443862 (closed)

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backend	@j_lar (UTC+1, 1 hour ahead of author)	@mwoolf (UTC+0, same timezone as author)

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@bauerdominic Would you mind taking an initial look at this?

Thanks for fixing this @sashi_kumar, LGTM! @Andyschoenen could you take the maintainer review please?

requested review from @bauerdominic

mentioned in merge request !145825 (merged)

mentioned in merge request !146313 (closed)

approved this merge request

requested review from @Andyschoenen and removed review request for @bauerdominic

added pipeline:mr-approved label

👋 @bauerdominic, thanks for approving this merge request.

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E2E Test Result Summary

allure-report-publisher generated test report!

e2e-test-on-gdk: ✅ test report for d4b612d7

expand test summary

+------------------------------------------------------------------+
|                          suites summary                          |
+-------------+--------+--------+---------+-------+-------+--------+
|             | passed | failed | skipped | flaky | total | result |
+-------------+--------+--------+---------+-------+-------+--------+
| Create      | 8      | 0      | 3       | 0     | 11    | ✅     |
| Monitor     | 4      | 0      | 0       | 0     | 4     | ✅     |
| Govern      | 66     | 0      | 0       | 0     | 66    | ✅     |
| Plan        | 4      | 0      | 0       | 0     | 4     | ✅     |
| Package     | 0      | 0      | 1       | 0     | 1     | ➖     |
| Data Stores | 2      | 0      | 0       | 0     | 2     | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+
| Total       | 84     | 0      | 4       | 0     | 88    | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+

e2e-package-and-test: ✅ test report for d4b612d7

expand test summary

+------------------------------------------------------------------+
|                          suites summary                          |
+-------------+--------+--------+---------+-------+-------+--------+
|             | passed | failed | skipped | flaky | total | result |
+-------------+--------+--------+---------+-------+-------+--------+
| Govern      | 272    | 0      | 19      | 0     | 291   | ✅     |
| Create      | 153    | 0      | 19      | 0     | 172   | ✅     |
| Plan        | 8      | 0      | 0       | 0     | 8     | ✅     |
| Monitor     | 8      | 0      | 0       | 0     | 8     | ✅     |
| Package     | 0      | 0      | 2       | 0     | 2     | ➖     |
| Data Stores | 4      | 0      | 0       | 0     | 4     | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+
| Total       | 445    | 0      | 40      | 0     | 485   | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+

mentioned in incident gitlab-com/gl-infra/production#17690 (closed)

Closing this due to gitlab-com/gl-infra/production#17690 (closed) & the revert: !146386 (merged)

closed

mentioned in merge request !146562 (merged)