Additional embedded apps for version 1.27.0 (#815)

* Create widget.radio_empty.py

* Use st.rerun in embedded apps

* Add placeholder to st.number_input example

Author: sfc-gh-dmatthews

python/api-examples-source/forms.form_process3.py

@@ -18,4 +18,4 @@
 # a second rerun when the form is submitted to update the value above.
 st.session_state.sum = a + b
 if submit:
-    st.experimental_rerun()
+    st.rerun()

python/api-examples-source/widget.number_input_empty.py

@@ -1,4 +1,4 @@
 import streamlit as st
 
-number = st.number_input("Insert a number", value=None)
+number = st.number_input("Insert a number", value=None, placeholder="Type a number...")
 st.write("The current number is ", number)

python/api-examples-source/widget.radio_empty.py

@@ -0,0 +1,9 @@
+import streamlit as st
+
+genre = st.radio(
+    "What's your favorite movie genre",
+    [":rainbow[Comedy]", "***Drama***", "Documentary :movie_camera:"],
+    index=None,
+)
+
+st.write("You selected:", genre)