sqlalchemy: `Sequence.start` incorrectly set to 1 when `minvalue` is set

Describe the bug

SQLAlchemy 1.4 will set Sequence.start to 1 if it is None during compile(). This is incorrect because the sequence may have a minvalue > 1, which is incompatible with start == 1.

To Reproduce

import sqlalchemy as sa
from sqlalchemy.dialects import postgresql

def dump(sql, *multiparams, **params):
    print(sql.compile(dialect=engine.dialect))

engine = sa.create_mock_engine('postgresql://', dump)

seq = sa.Sequence("my_seq", minvalue=1001)
print(f"{seq.start=}")
seq.create(engine)
print(f"{seq.start=}")

Error

Output:

seq.start=None
CREATE SEQUENCE my_seq START WITH 1 MINVALUE 1001
seq.start=1

Running that in Postgres will give:

START value (1) cannot be less than MINVALUE (1001)

Versions

• OS: Arch Linux
• Python: 3.9.7
• SQLAlchemy: 1.4.25 (SQLAlchemy-1.4.25-cp39-cp39-manylinux_2_5_x86_64.manylinux1_x86_64.manylinux_2_17_x86_64.manylinux2014_x86_64.whl)
• Database: PostgresSQL 13.4
• DBAPI (eg: psycopg, cx_oracle, mysqlclient): N/A

Additional context

This is a regression from 1.3. I think it is due to this code.