Resolve "Réorganisation criterion (null, not null)"

Closes #41 (closed)

client/src/app/instance/search/components/criteria/search-type/abstract-search-type.component.ts

-import { Directive, Input } from '@angular/core';
+/**
+ * This file is part of Anis Client.
+ *
+ * @copyright Laboratoire d'Astrophysique de Marseille / CNRS
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+import { Directive } from '@angular/core';
 import { FormGroup } from '@angular/forms';
 
 import { Attribute } from 'src/app/metamodel/models';
@@ -12,6 +21,7 @@ export abstract class AbstractSearchTypeComponent {
 
     form: FormGroup;
     operators = searchTypeOperators;
+    nullOrNotNull: string = '';
 
     constructor() { }
 
@@ -26,6 +36,7 @@ export abstract class AbstractSearchTypeComponent {
         } else {
             this.form.enable();
             this.form.reset();
+            this.nullOrNotNull = '';
         }
     }