Planner panics on virtual field

[Micah Wylde]

This query produces a panic in the planner:

create table logs (
    value TEXT NOT NULL,
    parsed TEXT GENERATED ALWAYS AS (value)
) with (
    connector = 'sse',
    endpoint = 'http://host.docker.internal:9563/sse',
    format = 'raw_string'
);

 
SELECT * from logs;

2023-12-01T05:53:23.641080Z ERROR arroyo_server_common: panicked at arroyo-sql/src/expressions.rs:1925:13:
internal error: entered unreachable code: invalid cast from Utf8 to Utf8 panic.file="arroyo-sql/src/expressions.rs" panic.line=1925 panic.column=13

This appears to be due to the NOT NULL to nullable conversion happening.