fix: handle case when no PCF is found in PCFSelection

[DBGR18]

fixing issue 1046

This pull request adds a safety check to the PCFSelection method in nrf_service.go to handle cases where no PCF instances are found, preventing potential nil pointer errors.

Error handling improvement:

• Added a check to return an error if res is nil or if there are no PCF instances found in res.SearchResult.NfInstances within the PCFSelection method of nrf_service.go.

[Copilot]

Pull request overview

This PR hardens SMF’s NRF consumer PCF discovery path by preventing a panic when NRF discovery returns no PCF instances (or a nil response), aligning PCF selection behavior with existing CHF selection handling.

Changes:

• Added a guard in PCFSelection to return a clear error when the NRF discovery response is nil or contains zero PCF instances.
• Prevented potential out-of-bounds / nil dereference when assigning SelectedPCFProfile.

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[d11nn]

@Alonza0314 LGTM