fix(startup): set full_screen when in ex_mode

Problem Description: In ex_mode, the default_grid.chars are not allocated, and subsequently,
the w_grid.target in curwin is not allocated to default_grid in update_screen. This leads to
a null pointer crash when the completion function is executed in ex_mode.

Solution: Set full_screen when in ex_mode to ensure that default_grid is allocated.

src/nvim/main.c

@@ -363,7 +363,7 @@ int main(int argc, char **argv)
 
   setbuf(stdout, NULL);  // NOLINT(bugprone-unsafe-functions)
 
-  full_screen = !silent_mode;
+  full_screen = !silent_mode || exmode_active;
 
   // Set the default values for the options that use Rows and Columns.
   win_init_size();