OZG-6720 set empty string as form value for patching

alfa-client/libs/admin/settings/src/lib/organisationseinheit/organisationseinheit-form-container/organisationseinheit-form/organisationseinheit.formservice.spec.ts

 import { AdminOrganisationsEinheitResource } from '@admin-client/admin-settings';
-import { StateResource, createStateResource } from '@alfa-client/tech-shared';
+import { EMPTY_STRING, StateResource, createStateResource } from '@alfa-client/tech-shared';
 import { Mock, mock, useFromMock } from '@alfa-client/test-utils';
 import { FormBuilder } from '@angular/forms';
+import { faker } from '@faker-js/faker';
 import { of } from 'rxjs';
 import { createAdminOrganisationsEinheitResource } from '../../../../../test/organisations-einheit/organisations-einheit';
 import { OrganisationsEinheitService } from '../../organisationseinheit.service';
@@ -39,4 +40,23 @@ describe('OrganisationsEinheitFormService', () => {
       expect(organisationsEinheitService.patch).toHaveBeenCalledWith(organisationsEinheitResource.settings);
     });
   });
+
+  describe('buildPatchedOrganisationsEinheitSettings', () => {
+    it('should return empty string', () => {
+      service.form.controls[OrganisationsEinheitFormService.ORGANISATIONSEINHEIT_SIGNATUR_FIELD].setValue(null);
+
+      const signatur: string = service.buildPatchedOrganisationsEinheitSettings().signatur;
+
+      expect(signatur).toEqual(EMPTY_STRING);
+    });
+
+    it('should return signatur', () => {
+      const formValueSignatur: string = faker.lorem.word(2);
+      service.form.controls[OrganisationsEinheitFormService.ORGANISATIONSEINHEIT_SIGNATUR_FIELD].setValue(formValueSignatur);
+
+      const signatur: string = service.buildPatchedOrganisationsEinheitSettings().signatur;
+
+      expect(signatur).toEqual(formValueSignatur);
+    });
+  });
 });

alfa-client/libs/admin/settings/src/lib/organisationseinheit/organisationseinheit-form-container/organisationseinheit-form/organisationseinheit.formservice.ts

-import { AdminOrganisationsEinheitResource } from '@admin-client/admin-settings';
+import { AdminOrganisationsEinheitResource, AdminOrganisationsEinheitSettings } from '@admin-client/admin-settings';
 import { AbstractFormService, EMPTY_STRING, StateResource } from '@alfa-client/tech-shared';
 import { Injectable } from '@angular/core';
 import { FormControl, UntypedFormBuilder, UntypedFormGroup } from '@angular/forms';
@@ -23,7 +23,12 @@ export class OrganisationsEinheitFormService extends AbstractFormService {
   }
 
   protected doSubmit(): Observable<StateResource<AdminOrganisationsEinheitResource>> {
-    return this.organisationsEinheitService.patch(this.getFormValue());
+    return this.organisationsEinheitService.patch(this.buildPatchedOrganisationsEinheitSettings());
+  }
+
+  buildPatchedOrganisationsEinheitSettings(): AdminOrganisationsEinheitSettings {
+    const signatur: string = this.getFormValue().signatur ? this.getFormValue().signatur : '';
+    return { signatur };
   }
 
   protected getPathPrefix(): string {