Silence most -Wzero-as-null-pointer-constant diagnostics

Replace 0 by NULL and {0} by {}.

Omit a few cases that aren't so trivial to fix.

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>
Link: <https://software.codidact.com/posts/292718/292759#answer-292759>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

string/argz-delete.c

@@ -33,7 +33,7 @@ argz_delete (char **argz, size_t *argz_len, char *entry)
       if (*argz_len == 0)
 	{
 	  free (*argz);
-	  *argz = 0;
+	  *argz = NULL;
 	}
     }
 }