UI: Minor fix style and fix signing key validation

ui-ngx/src/app/modules/home/pages/admin/security-settings.component.html

@@ -180,6 +180,7 @@
             <mat-label translate>admin.jwt.signings-key</mat-label>
             <input matInput (focus)="markAsTouched()" required formControlName="tokenSigningKey"/>
             <button type="button"
+                    style="line-height: 32px"
                     matSuffix
                     mat-button
                     (click)="generateSigningKey()"

ui-ngx/src/app/modules/home/pages/admin/security-settings.component.ts

@@ -172,9 +172,12 @@ export class SecuritySettingsComponent extends PageComponent implements HasConfi
   }
 
   private base64Format(control: FormControl): { [key: string]: boolean } | null {
+    if (control.value === '' || control.value === 'thingsboardDefaultSigningKey') {
+      return null;
+    }
     try {
       const value = atob(control.value);
-      if (value.length < 32 && control.value !== 'thingsboardDefaultSigningKey') {
+      if (value.length < 32) {
         return {minLength: true};
       }
       return null;