Problem
=======
The GeoJSON standard specifies the following: "A complete GeoJSON data
structure is always an object (in JSON terms).". That means that if the input
to ST_GeomFromGeoJSON is NOT a JSON object, it is considered illegal. And as
with almost all other MySQL functions, if any of the input arguments to
ST_GeomFromGeoJSON is SQL NULL, we return SQL NULL.

The use case reported can be boiled down to the following statements for
simplicity:

  CREATE TABLE t1 (col1 JSON);
  INSERT INTO t1 VALUES ('{ "type": "Feature", "geometry": null,
"properties": {} }');
  ALTER TABLE t1 ADD COLUMN `col2` geometry GENERATED ALWAYS AS
(ST_GeomFromGeoJSON(col1->'$.geometry')) VIRTUAL;
  SELECT * FROM t1;

... or even simpler:

  SELECT ST_GeomFromGeoJSON(CAST('null' AS JSON));

The query "SELECT * FROM t1;" fails, because when the virtual column 'col2' tries to
extract the property 'geometry' from the JSON field, it gets a JSON NULL and
not a SQL NULL. Hence, it is considered as an illegal/invalid GeoJSON input.

Fix
===
Treat JSON NULL as a SQL NULL in ST_GeomFromGeoJSON, and thus return SQL NULL
from ST_GeomFromGeoJSON when the input value is a JSON NULL.

Also change Item_func_geomfromgeojson to set maybe_null always to true, since
a GeoJSON Feature object can cause ST_GeomFromGeoJSON to return SQL NULL.

(cherry picked from commit 42a9f760aa2f5e69cc0e72f899b55ad78dda459d)

Conflicts:
	mysql-test/suite/gis/r/geojson_functions.result
	mysql-test/suite/gis/t/geojson_functions.test
	sql/item_geofunc.cc