Adjusted IdentifierCreateDto to represent the requirements for DOIs

dbrepo-metadata-db/api/src/main/java/at/tuwien/api/identifier/IdentifierCreateDto.java

@@ -11,6 +11,7 @@ import lombok.Setter;
 
 import javax.validation.constraints.NotBlank;
 import javax.validation.constraints.NotNull;
+import javax.validation.constraints.NotEmpty;
 import java.util.List;
 
 @Data
@@ -58,6 +59,7 @@ public class IdentifierCreateDto {
     private Integer publicationYear;
 
     @NotNull
+    @NotEmpty
     private List<CreatorCreateDto> creators;
 
     @JsonProperty("related_identifiers")