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';
+import { Directive } from '@angular/core';
 import { FormGroup } from '@angular/forms';
 
 import { Attribute } from 'src/app/metamodel/models';
@@ -12,6 +12,7 @@ export abstract class AbstractSearchTypeComponent {
 
     form: FormGroup;
     operators = searchTypeOperators;
+    nullOrNotNull: string = '';
 
     constructor() { }