_decode_default_value returns string "TRUE"/"FALSE"/"NULL" instead of Python bool/None #836

When a column is defined with DEFAULT TRUE, DEFAULT FALSE, or DEFAULT NULL, SQLite's PRAGMA table_info returns the literal strings "TRUE", "FALSE", or "NULL". _decode_default_value does not handle these keyword literals and falls through to return the raw string instead of the corresponding Python object.

Effect on table.default_values:

import sqlite_utils
db = sqlite_utils.Database(memory=True)
db.execute("CREATE TABLE t (id INTEGER PRIMARY KEY, flag INTEGER DEFAULT TRUE, nullable TEXT DEFAULT NULL)")
print(db["t"].default_values)
# {'flag': 'TRUE', 'nullable': 'NULL'}   ← wrong; should be {'flag': True, 'nullable': None}

Effect on .transform():

Because default_values returns strings instead of Python booleans/None, the comparison at the transform step that decides whether a table rebuild is needed treats True != "TRUE", triggering an unnecessary table rebuild even when no defaults have changed.

Fix: add three branches to _decode_default_value before the float() attempt:

upper = value.upper()
if upper == "TRUE":
    return True
if upper == "FALSE":
    return False
if upper == "NULL":
    return None