fix start read widget on initialization

front/src/BookForm.vue

@@ -44,7 +44,7 @@
   }
 
   function onStartReadIconClick() {
-    if (data.value.startReadDate == null) {
+    if (!data.value.startReadDate) {
       data.value.startReadDate = new Date().toISOString().slice(0, 10);
       putStartReadDate(props.id, data.value.startReadDate);
     } else {

front/src/StartReadWidget.vue

@@ -11,7 +11,7 @@
   const today = new Date().toISOString().slice(0, 10);
 
   function computeParentClasses() {
-    if (props.startReadDate != null) {
+    if (props.startReadDate) {
       return "startdatewidget has-text-dark has-background-text"
     } else {
       return "startdatewidget"
@@ -24,9 +24,9 @@
   <div :class="computeParentClasses()">
     <BigIcon icon="BIconBook"
              :legend="props.legend"
-             :isSet="props.startReadDate != null"
+             :isSet="props.startReadDate ? true : false"
              @click="$emit('onIconClick')"/>
-    <div v-if="props.startReadDate != null" class="inputdate">
+    <div v-if="props.startReadDate" class="inputdate">
       <div class="ontopofinput">
         <label class="datelabel" for="startread">
           {{$t('startreadwidget.started')}}