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-extract.c

@@ -30,6 +30,6 @@ __argz_extract (const char *argz, size_t len, char **argv)
       argz += part_len + 1;
       len -= part_len + 1;
     }
-  *argv = 0;
+  *argv = NULL;
 }
 weak_alias (__argz_extract, argz_extract)