databases/cego: 2.20.22 -> 2.20.23

• SQL optimizer improvement was added. In case of select conditions with "a is null", neither index nor btree was used. Just for "a = null", the index was used. This has been fixed.
• Added fix for btree null value handling. A dedicated null value flag was introduced for each btree value ( class CegoBTreeValue ) to handle null values.

Submitted by: lemke@lemke-it.com