refactor: replace '\0' with NUL

src/nvim/base64.c

@@ -4,6 +4,7 @@
 #include <string.h>
 
 #include "auto/config.h"  // IWYU pragma: keep
+#include "nvim/ascii_defs.h"
 #include "nvim/base64.h"
 #include "nvim/memory.h"
 
@@ -125,7 +126,7 @@ char *base64_encode(const char *src, size_t src_len)
     dest[out_i] = '=';
   }
 
-  dest[out_len] = '\0';
+  dest[out_len] = NUL;
 
   return dest;
 }