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/envz.c

@@ -50,7 +50,7 @@ envz_entry (const char *envz, size_t envz_len, const char *name)
 	envz++, envz_len--;	/* skip '\0' */
     }
 
-  return 0;
+  return NULL;
 }
 libc_hidden_def (envz_entry)
 
@@ -67,7 +67,7 @@ envz_get (const char *envz, size_t envz_len, const char *name)
       if (*entry)
 	entry++;
       else
-	entry = 0;		/* A null entry.  */
+	entry = NULL;		/* A null entry.  */
     }
   return entry;
 }