empty string in composite data types

Submitted by: Christian von Kietzell

Psycopg incorrectly returns empty strings in composite data types as None instead of ''.

For example:

sql> CREATE TYPE testtype AS (id INT, label TEXT);

python> import psycopg2
python> import psycopg2.extras
python> db = psycopg2.connect(...)
python> caster = psycopg2.extras.register_composite('testtype', db)
python> cursor = db.cursor()
python> cursor.execute("SELECT (1, '')::testtype")
python> cursor.fetchone()[0]
testtype(id=1, label=None)