Fix flaky 25_downstream_pipelines fixture

What does this MR do?

ee/db/fixtures/development/25_downstream_pipelines.rb sometimes fails when target_pipeline is nil. This could happen when the seed size of the projects is specified by SIZE environment variable. We sometimes specify 1 to the size, so that there is only one project created in the instance initially, which fails the 25_downstream_pipelines expectation.

We should skip the process if the target_pipeline is not found.

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