minor fixes

src/main/java/de/helmholtz/marketplace/cerebrum/controller/ImageController.java

@@ -66,7 +66,7 @@ public class ImageController
     @GetMapping(path = "")
     public Iterable<Image> getImages(
             @Parameter(description = "specify the name of the image to search for")
-            @RequestParam(value = "name") String name,
+            @RequestParam(value = "name", defaultValue = "") String name,
             @Parameter(description = "specify the page number")
             @RequestParam(value = "page", defaultValue = "0") @Min(0) Integer page,
             @Parameter(description = "limit the number of records returned in one page")

src/main/java/de/helmholtz/marketplace/cerebrum/entity/MarketUser.java

@@ -5,7 +5,6 @@ import lombok.AccessLevel;
 import lombok.Getter;
 import lombok.Setter;
 import org.springframework.data.annotation.Id;
-import org.springframework.data.mongodb.core.mapping.DBRef;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.lang.Nullable;