Fix pipeline success requirement for MRs in security policy projects

What does this MR do and why?

In Disable pipeline must succeed setting for secur... (!162462 - merged), we fixed a bug with security policy projects which inherits only_allow_merge_if_pipeline_succeeds from the group settings. But after the fix, updating the setting for the security policy project was not possible as it was set to false by default.

This MR fixes the bug by taking the value of the project instead of returning false by default.

References

• #552085 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Create a group/sub-group and disable Pipeline must succeed setting from Settings->Merge requests
• Create a security policy for the group/sub-group and verify that the security policy project is created
• Go to security policy project -> Settings -> Merge requests and select Pipeline must succeed and save. Verify that the settings is enabled.
• Verify that MR created in the policy project cannot be merged without a pipeline

MR acceptance checklist

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Related to #552085 (closed)