Resolve "Réorganisation criterion (null, not null)"

Closes #41 (closed)

client/src/app/instance/search/components/criteria/search-type/checkbox.component.html

 <form [formGroup]="form" novalidate>
     <div class="row form-group">
         <div class="col-md-3 col-sm-auto pr-sm-1 mb-1 mb-lg-0">
-            <div class="operator_readonly">in</div>
+            <select class="custom-select" (change)="labelOnChange($event.target.value)">
+                <option [value]="">In</option>
+                <option [value]="'nl'">Null</option>
+                <option [value]="'nnl'">Not Null</option>
+            </select>
         </div>
         <div class="w-100 d-block d-sm-none"></div>
         <div class="col pl-sm-1" formArrayName="checkboxes">