As Gorgious commented, this is the code:
# The draw_item function is called for each item of the collection that is visible in the list.
# data is the RNA object containing the collection,
# item is the current drawn item of the collection,
# icon is the "computed&...
Thanks to a comment on this answer I was pointed in the right direction.
Internally, the 'EMPTY' string is converted into SPACE_EMPTY. SPACE_EMPTY is used in the C code for various purposes, including as the space type for file dropboxes and for newly created empty screens. It has many other uses related to versioning and detecting uninitialized data; but ...
I think it doesn't serve any purpose in the context of panels.
It looks like it is mainly (only) used in the keymap definitions.
From what I could tell it means it's supposed to be shared by several space types.
For instance every space type can have Window shortcuts, Screen shortcuts, etc.
And Clip, Frame manipulation, Mask editing, Grease Pencil, etc. do ...
Good news ! You have done 99% of the work :)
You just need to display the right search operator in your panel, and call your operator after having selected the right item in the search popup.
bl_idname = "object.search_popup"
bl_label = "Material Renamer"
bl_property = "...