[ruby/rdoc] fix: Fix NoMethodError for tokens_to_s method

Calling `tokens_to_s` gets an error if `token_stream` is nil:

```
undefined method `compact' for nil:NilClass (NoMethodError)
```

So, fall back to an empty array if `@token_stream` is nil.

452e4a2600

lib/rdoc/token_stream.rb

@@ -105,7 +105,7 @@ module RDoc::TokenStream
   # Current token stream
 
   def token_stream
-    @token_stream
+    @token_stream || []
   end
 
   ##

test/rdoc/test_rdoc_token_stream.rb

@@ -53,5 +53,14 @@ class TestRDocTokenStream < RDoc::TestCase
     end.new
 
     assert_equal "foo 'bar'", foo.tokens_to_s
+
+    foo = Class.new do
+      include RDoc::TokenStream
+
+      def initialize
+        @token_stream = nil
+      end
+    end.new
+    assert_equal "", foo.tokens_to_s
   end
 end