Can a @ManyToOne JPA relation be null?

You need to set:

@ManyToOne(optional = true, fetch = FetchType.LAZY)

not optional=false.

The @Column(nullable=true) is to instruct the DDL generation tool to include a NULL SQL column type constraint.

For more on optional vs nullable, check out this StackOverflow answer.

try this:

@JoinColumn(name = "subType_id", nullable = true)