With default spring.jpa.properties.hibernate.id.new_generator_mappings=false generator and
@GeneratedValue (default GenerationType.AUTO) strategy Hibernate autogenerate scheme:
{{
create sequence hibernate_sequence start with 1 increment by 1;
create table users (id integer not null, ...);
}}
So id is not autoincrement and JDBC script INSERT failed with
org.h2.jdbc.JdbcSQLException:NULL not allowed for column "ID"; SQL statement

According H2 grammar
create table users (id int default hibernate_sequence.nextval primary key);
must be generated for id autoincremening