fix: avoid errors when depending on rules that have no dependencies

doxygen/doxygen.bzl

diff --git a/doxygen/doxygen.bzl b/doxygen/doxygen.bzl
index c2494ea..0376754 100644
--- a/doxygen/doxygen.bzl
+++ b/doxygen/doxygen.bzl
@@ -37,7 +37,7 @@
 
     # Collect transitive files from dependencies
     transitive_files = []
-    for dep in ctx.rule.attr.deps:
+    for dep in ctx.rule.attr.deps if hasattr(ctx.rule.attr, "deps") else []:
         if TransitiveSourcesInfo in dep:
             transitive_files.append(dep[TransitiveSourcesInfo].srcs)