Add restriction to prevent remove all the lines in a personal expense sheet

A personal expense sheet is bound to a resource, and the resource is specified
in the lines. So, if user removes all the lines we'll be losing that
relationship.

FEA: ItEr76S28UserDashboard

libreplan-business/src/main/java/org/libreplan/business/expensesheet/entities/ExpenseSheet.java

@@ -232,4 +232,13 @@ public class ExpenseSheet extends IntegrationEntity implements IHumanIdentifiabl
         return !personal;
     }
 
+    @AssertTrue(message = "a personal expenses must have at least one line")
+    public boolean checkConstraintPersonalExpenseMustHaveAtLeastOneLine() {
+        if (!personal) {
+            return true;
+        }
+
+        return !expenseSheetLines.isEmpty();
+    }
+
 }