Check for 'deps' attr in scala_doc rule (bazel-contrib#1740)

* Check for 'deps' attr in scala_doc

* code review suggestions

Author: vinnybod

scala/private/rules/scala_doc.bzl

@@ -34,7 +34,7 @@ def _scaladoc_aspect_impl(target, ctx, transitive = True):
 
     macro_classpath = []
 
-    for dependency in ctx.rule.attr.deps:
+    for dependency in getattr(ctx.rule.attr, "deps", []):
         if ScalaInfo in dependency and dependency[ScalaInfo].contains_macros:
             macro_classpath.append(dependency[JavaInfo].transitive_runtime_jars)