ArcGIS Pro 2.6.8; Oracle 18c; 10.7.1 EGDB; SDE.ST_GEOMETRY
Regarding the isEmpty ST_GEOMETRY SQL function:
ST_IsEmpty returns 1 (Oracle and SQLite) or t (PostgreSQL) if the ST_Geometry object is empty; otherwise, it returns 0 (Oracle and SQLite) or f (PostgreSQL).
It seems like an empty geometry can either be:
- Shape is not null, and geometry is not null (geometry is Nil/zero-vertex), or
- Shape is not null, but geometry is null
In other words, the shape is not null, and the geometry is either zero-vertex or null.
Is that correct?
SQL testing:

select
objectid,
shape,
(shape).points as geometry,
sde.st_area(shape) as area,
sde.st_isempty(shape) as is_empty,
comment3,
(shape).entity,
sde.st_entity(shape),
(shape).numpts
from
infrastr.inf_record_sp
Related: Idea - Define "is empty" in the ST_GEOMETRY documentation