SERVER-120626 added a mutable getter to `getSubPipelines` via `getMutableSubPipelines`. However, this can potentially return a reference to a statically allocated object `kNoSubPipeline`.

A more idiomatic approach would be instead return either a shared pointer or a `boost::intrusive_ptr` from `getSubPipelines` and `getMutableSubPipelines` (const vs non-const, respectively), or nullptr if there are no subpipelines.

This would require also changing LiteParsedDocumentSourceNestedPipelines to track its `_pipelines` member as a pointer value, as well as all callsites to handle the potentially null value.