lib/mtree: drop redundant name checks

This drops several NULL checks against filename input argument.
Those checks are both redundant (as filename validation already
checks for that) and dangerous (as they return early without
setting an error value).

src/libostree/ostree-mutable-tree.c

@@ -303,8 +303,6 @@ ostree_mutable_tree_replace_file (OstreeMutableTree *self,
                                   const char        *checksum,
                                   GError           **error)
 {
-  g_return_val_if_fail (name != NULL, FALSE);
-
   if (!ot_util_filename_validate (name, error))
     return FALSE;
 
@@ -338,8 +336,6 @@ ostree_mutable_tree_remove (OstreeMutableTree *self,
                             gboolean           allow_noent,
                             GError           **error)
 {
-  g_return_val_if_fail (name != NULL, FALSE);
-
   if (!ot_util_filename_validate (name, error))
     return FALSE;
 
@@ -374,8 +370,6 @@ ostree_mutable_tree_ensure_dir (OstreeMutableTree *self,
                                 OstreeMutableTree **out_subdir,
                                 GError           **error)
 {
-  g_return_val_if_fail (name != NULL, FALSE);
-
   if (!ot_util_filename_validate (name, error))
     return FALSE;