Having maybe_null flag unset for geometry/spatial functions leads to
wrong Item_func_isnull::val_int()'s results.
Fix: set maybe_null flag and add is_null() methods.


mysql-test/r/gis.result:
  Fix for bug #26038: X() value of empty NOT NULL POINT is neither NULL nor NOT NULL
    - test result.
mysql-test/t/gis.test:
  Fix for bug #26038: X() value of empty NOT NULL POINT is neither NULL nor NOT NULL
    - test case.
sql/item_geofunc.cc:
  Fix for bug #26038: X() value of empty NOT NULL POINT is neither NULL nor NOT NULL
    - set maybe_null flag for Item_geometry_func and Item_func_as_wkt.
    - moved length check to the beginnig of the 
      Item_func_spatial_collection::val_str() to affect geometry 
      collection objects at once.
    - changed Item_func_isempty::val_int() and Item_func_issimple::val_int()
      to properly handle null_value.
sql/item_geofunc.h:
  Fix for bug #26038: X() value of empty NOT NULL POINT is neither NULL nor NOT NULL
    - set maybe_null flag for geometry/spatial functions.
    - added is_null() to Item_geometry_func and Item_func_spatial_rel
      classes.
sql/spatial.cc:
  Fix for bug #26038: X() value of empty NOT NULL POINT is neither NULL nor NOT NULL
    - changed return type of Geometry::create_from_wkb() to be 
      consistent with Geometry::create_from_wkt(), now it returns
      Geometry object or NULL in case of error.
sql/spatial.h:
  Fix for bug #26038: X() value of empty NOT NULL POINT is neither NULL nor NOT NULL
    - changed return type of Geometry::create_from_wkb() to be 
      consistent with Geometry::create_from_wkt(), now it returns
      Geometry object or NULL in case of error.