fix #4315

Closes #4315

Merge request studip/studip!3173

resources/vue/components/courseware/structural-element/CoursewareStructuralElement.vue

@@ -337,6 +337,7 @@
                                             <translate>Niveau</translate><br />
                                             <translate>von</translate>
                                             <select v-model="currentElement.attributes.payload.difficulty_start">
+                                                <option value="">-</option>
                                                 <option
                                                     v-for="difficulty_start in 12"
                                                     :key="difficulty_start"
@@ -347,6 +348,7 @@
                                             </select>
                                             <translate>bis</translate>
                                             <select v-model="currentElement.attributes.payload.difficulty_end">
+                                                <option value="">-</option>
                                                 <option
                                                     v-for="difficulty_end in 12"
                                                     :key="difficulty_end"

resources/vue/components/courseware/structural-element/CoursewareStructuralElementDialogAdd.vue

@@ -131,12 +131,14 @@
                         {{ $gettext('Niveau') }}<br />
                         {{ $gettext('von') }}
                         <select v-model="difficulty_start">
+                            <option value="">-</option>
                             <option v-for="difficulty_start in 12" :key="difficulty_start" :value="difficulty_start">
                                 {{ difficulty_start }}
                             </option>
                         </select>
                         {{ $gettext('bis') }}
                         <select v-model="difficulty_end">
+                            <option value="">-</option>
                             <option v-for="difficulty_end in 12" :key="difficulty_end" :value="difficulty_end">
                                 {{ difficulty_end }}
                             </option>

resources/vue/components/courseware/unit/CoursewareShelfDialogAdd.vue

@@ -118,6 +118,7 @@
                         {{ $gettext('Niveau') }}<br />
                         {{ $gettext('von') }}
                         <select v-model="addWizardData.difficulty_start">
+                            <option value="">-</option>
                             <option
                                 v-for="difficulty_start in 12"
                                 :key="difficulty_start"
@@ -128,6 +129,7 @@
                         </select>
                         {{ $gettext('bis') }}
                         <select v-model="addWizardData.difficulty_end">
+                            <option value="">-</option>
                             <option
                                 v-for="difficulty_end in 12"
                                 :key="difficulty_end"