Consider using `@NotNull` annotations

[marcwrobel]

@NotNull annotations are not used in jbanking : so far keeping the library completely dependency-free was preferred. But this annotation helps static code analysis, can be really helpful in IDEs or with some languages like Kotlin.

Should we consider using @NotNull annotations ?

[marcwrobel]

A few links :

• https://stackoverflow.com/questions/4963300/which-notnull-java-annotation-should-i-use
• https://jspecify.dev/
• https://github.com/marcwrobel/jbanking/pull/223#discussion_r1000376897