doc: _scripts: boards: fix for boards without revisions

Commit 38b8790 introduced a pretty bad regression for boards without
revisions. Fix by correctly checking for actually undefined revisions

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

doc/_scripts/gen_boards_catalog.py

diff --git a/doc/_scripts/gen_boards_catalog.py b/doc/_scripts/gen_boards_catalog.py
index 40b516a..a22ad11 100755
--- a/doc/_scripts/gen_boards_catalog.py
+++ b/doc/_scripts/gen_boards_catalog.py
@@ -163,7 +163,7 @@
                 revision = board_info.get('revision', '')
 
                 board_target = board_name
-                if revision is not None:
+                if revision != '':
                     board_target = f"{board_target}@{revision}"
                 if qualifier:
                     board_target = f"{board_target}/{qualifier}"