Avoid serializing NULL string through msgpack

Attempting to serialize a NULL string through msgpack results in
msgpack_sbuffer_write attempting to memcpy from a NULL pointer, which is
undefined behavior.

src/nvim/shada.c

@@ -1575,7 +1575,9 @@ static char *shada_filename(const char *file)
     do { \
       const String s_ = (s); \
       msgpack_pack_bin(spacker, s_.size); \
-      msgpack_pack_bin_body(spacker, s_.data, s_.size); \
+      if (s_.size > 0) { \
+        msgpack_pack_bin_body(spacker, s_.data, s_.size); \
+      } \
     } while (0)
 
 /// Write single ShaDa entry