ex_getln: Return something from hist_type2char without asserts

Otherwise QuickBuild shows a warning that control may reach end of non-void
function.

src/nvim/ex_getln.c

@@ -4783,6 +4783,7 @@ int hist_type2char(int type)
       assert(false);
     }
   }
+  return NUL;
 }
 
 /*