ci: Enable tests with no platform specified

After `.platform` mechanism was introduced in #159, it silently filters
out tests with no `platform` specified. Enable them to execute by adding
`platform is not None` predicate preceeds allowlist check.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

.buildkite/autogenerate_pipeline.py

@@ -328,7 +328,7 @@ def build(self, input, platform_allowlist):
 
             for platform in platforms:
                 # Filter test enabled in platform_allowlist
-                if platform not in platform_allowlist:
+                if platform is not None and platform not in platform_allowlist:
                     # Skip disabled platform
                     continue