supervisor: Always allocate at least a 1x1 terminal

Otherwise, out of range writes would occur in tilegrid_set_tile, causing a safe mode reset.
```
Hardware watchpoint 6: -location *stack_alloc->ptr

Old value = 24652061
New value = 24641565
0x000444f2 in common_hal_displayio_tilegrid_set_tile (self=0x200002c8 <supervisor_terminal_text_grid>, x=1, y=1, tile_index=0 '\000')
    at ../../shared-module/displayio/TileGrid.c:236
236	    if (!self->partial_change) {
(gdb)
```

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