Replace null publication date in Book object

Hi Team,
Currently, the Book object is initialized with the publication date set to null.
For better data integrity and functionality,
it would be beneficial to include a publication date when creating a Book object.

https://github.com/JetBrains/build-UI-using-Swing-sample-project/blob/3d6f3ba095c1c4259969cea1a27d67e07d27dc98/src/com/example/library/forms/BookEditorExample.java#L67

https://github.com/JetBrains/build-UI-using-Swing-sample-project/blob/3d6f3ba095c1c4259969cea1a27d67e07d27dc98/src/com/example/library/Book.java#L11C5-L16C6

Proposed Solution:

1. Modify the Book class to accept a publication date parameter.
2. Update instances where the Book object is created to include the publication date.
3. Update the sysout line of the Main class to print the publication date in the console.

Checklist
[Y] I have read and followed the project's code of conduct.
[Y] I have searched for similar issues before creating this one.
[Y] I have provided all the necessary information to understand and reproduce the issue.
[Y] I am willing to contribute to the resolution of this issue.

Thank you,
Subhojit