[lldb/Plugins] Use default initializers for StructuredData::*SP (NFC)

This patch replaces the ScriptedProcessInterface getters to return
default initializers for StructureData shared pointers instead of
returning a null pointer.

Differential Revision: https://reviews.llvm.org/D137359



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>