Fix extract_function when the extracted code defines no variable

If the extracted lines define no name, the new function ended with a
bare "return " and the call site became " = func(v1)". Both are
invalid Python.

Only add the final return line and the assignment when there is at
least one variable to return. Otherwise call the function as a
statement.

test/refactor/extract_function.py

@@ -314,6 +314,22 @@ def x(v1):
     #bar
     return x
 x
+# -------------------------------------------------- with-range-func-no-output
+def x(v1):
+    #? 2 text {'new_name': 'func', 'until_line': 4}
+    print(v1)
+    print(v1 + 1)
+    return v1
+# ++++++++++++++++++++++++++++++++++++++++++++++++++
+def func(v1):
+    print(v1)
+    print(v1 + 1)
+
+
+def x(v1):
+    #? 2 text {'new_name': 'func', 'until_line': 4}
+    func(v1)
+    return v1
 # -------------------------------------------------- in-class-range-1
 class X1:
     #? 9 text {'new_name': 'f', 'until_line': 4}