PVS/V1032: pointer cast to a more strictly aligned type

Rework-of: ea7491586f
Helped-by: Björn Linse <bjorn.linse@gmail.com>

- The old (Vim) use of (char_u **)"" before ea7491586f is garbage,
  which hints that this value was never used.
- The necessary condition is next to the NULL assigmnent, the pointer
  would only be started to be accessed, if the length assignment next to
  it is also changed.

src/nvim/ex_getln.c

@@ -4702,7 +4702,7 @@ ExpandFromContext (
     return ret;
   }
 
-  *file = &vim_emptystr;
+  *file = NULL;
   *num_file = 0;
   if (xp->xp_context == EXPAND_HELP) {
     /* With an empty argument we would get all the help tags, which is