Handle invalid option tag with name only

We have some of these in our codebase:

```
@option :foo
```

Handle them gracefully, even though they are invalid.

lib/yard/tags/default_factory.rb

@@ -73,6 +73,9 @@ def parse_tag_with_title_and_text(tag_name, text)
       end
 
       def parse_tag_with_types_name_and_default(tag_name, text)
+        if text.nil?
+          return nil
+        end
         # Can't allow () in a default tag, otherwise the grammar is too ambiguous when types is omitted.
         open = TYPELIST_OPENING_CHARS.delete('(')
         close = TYPELIST_CLOSING_CHARS.delete(')')

spec/tags/default_factory_spec.rb

@@ -164,5 +164,10 @@ def parse_options(text)
       expect(t.pair).to be_instance_of(Tags::DefaultTag)
       expect(t.pair.name).to eq "key"
     end
+
+    it "allows name only" do
+      t = parse_options("xyz")
+      expect(t.pair).to be(nil)
+    end
   end
 end