Automatic default_value in radiobutton

**The (small) problem**
```
import dearpygui.dearpygui as dpg

dpg.create_context()
dpg.create_viewport(title="Viewport")

def print_values():
print("radio1: ", dpg.get_value("radio1"))
print("radio2: ", dpg.get_value("radio2"))

with dpg.window(label="Window", tag="window"):
dpg.add_button(label="Print values", callback=print_values)
dpg.add_radio_button(items=['Yes', 'No'], tag="radio1")
dpg.add_radio_button(items=['Yes', 'No'], tag="radio2", default_value='Yes')

dpg.set_primary_window("window", True)
dpg.setup_dearpygui()
dpg.show_viewport()
dpg.start_dearpygui()
dpg.destroy_context()
```

If you run this example and do nothing except for clicking print values. Then you will see that ``dpg.get_value("radio1")`` returns ``None``. It is not a big problem, since you can just specify default_value. But, it can be confusing since the first option is always selected in the GUI even if default_value is not specified.

**Preferred solution**
I think it would make sense in this situation to return the first list item unless default value is specified.
```
content = [1, 2, 3, 4, 5]

dpg.add_radio_button(items=content) # add default_value=content[0] automatically
```

**Other solutions**
Just have nothing selected in the GUI if default_value is unspecified.

**Note**
This is my first feature request. If it has wrong tags or something, please let me know.