Post-auth null pointer dereference when aggregating against a view with empty search pipeline

An authenticated user can crash mongod when running $rankFusion or $scoreFusion with an empty pipeline on a view.

When resolving a view, the server inspects the aggregation pipeline to determine whether it begins with an Atlas Search stage. For $rankFusion and $scoreFusion, this inspection reads the first element on each stage’s input pipeline array without first verifying that the array is non-empty. Supplying an empty pipeline causes a null pointer dereference and crashes the server.

This issue affects MongoDB Server 8.2 versions prior to 8.2.7.