Fix default value for directory

Author: Saurye

src/ngx-file-drop/ngx-file-drop.component.html

@@ -8,11 +8,11 @@
       type="file" 
       #fileSelector 
       [accept]="accept" 
-      [attr.directory]="directory" 
-      [attr.webkitdirectory]="directory"
-      [attr.mozdirectory]="directory"
-      [attr.msdirectory]="directory"
-      [attr.odirectory]="directory"
+      [attr.directory]="directory || undefined" 
+      [attr.webkitdirectory]="directory || undefined"
+      [attr.mozdirectory]="directory || undefined"
+      [attr.msdirectory]="directory || undefined"
+      [attr.odirectory]="directory || undefined"
       [multiple]="multiple"
       (change)="uploadFiles($event)" 
       class="ngx-file-drop__file-input"