ENHO: check existence before deletion

With this commit we add an existence check the delete method
of ENHO serializer because some enhancements, like implicit source code
enhancements of classes, are deleted implicitly.

src/objects/zcl_abapgit_object_enho.clas.abap

@@ -116,6 +116,9 @@ CLASS ZCL_ABAPGIT_OBJECT_ENHO IMPLEMENTATION.
     DATA: lv_enh_id     TYPE enhname,
           li_enh_object TYPE REF TO if_enh_object.
 
+    IF zif_abapgit_object~exists( ) = abap_false.
+      RETURN.
+    ENDIF.
 
     lv_enh_id = ms_item-obj_name.
     TRY.