doc: _scripts: boards: fix condition for revision

Revision might be `0` so update the condition to not inadvertently
exclude boards with such revision number.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/_scripts/gen_boards_catalog.py

@@ -138,7 +138,7 @@ def gather_board_devicetrees(twister_out_dir):
                 revision = board_info.get('revision', '')
 
                 board_target = board_name
-                if revision:
+                if revision is not None:
                     board_target = f"{board_target}@{revision}"
                 if qualifier:
                     board_target = f"{board_target}/{qualifier}"