On a non-mandatory foreign key is left empty, the following error message is displayed at record saving or in validation context:

It prevents the user from saving the record and raises an error in the validation report.

This message shows up when the primary key of the target table is allowed to be empty, because of a regular expression pattern.
The solution is to modify the pattern of that primary key so that it forbids empty strings.

Example: [a-zA-Z]* → [a-zA-Z]+ (zero or more letters → one or more letters)

Please refer to the empty string management documentation for more information.