tests

Add `Value::as_null` #2256

Job	Run time
Build `sea-query-binder` (async-std, rustls)	58s
Build `sea-query`	28s
Build `sea-query-postgres`	26s
Clippy	27s
Build `sea-query-rusqlite`	23s
Rustfmt	16s
Unit Test	0s
Derive Tests	0s
MySQL	0s
PostgreSQL	0s
MariaDB	0s
Any	0s
SQLite	0s
	2m 58s

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