validation: cover nil/None return from lua_func/py_func

Two new steps per language: function returning nothing and function
returning explicit nil/None. Both tagged $(test)_PASS — they would
have failed before the lua nil fix (Lua side reported nil result as
error). Python side already worked but is covered for parity.

test/validation/items/lua_func/lua_func.lua

@@ -49,4 +49,12 @@ function module.test_delgd()
     return 0
 end
 
+function module.return_nothing()
+    -- Returns no value: ret is nil but no error.
+end
+
+function module.return_explicit_nil()
+    return nil
+end
+
 return module

test/validation/items/lua_func/test.tum

@@ -186,6 +186,18 @@
     file: $(test_path)$(psep)lua_func.lua
     func_name: test_delgd
 
+- lua_func:
+    name: function returning nothing should succeed
+    key: $(test)_PASS
+    file: $(test_path)$(psep)lua_func.lua
+    func_name: return_nothing
+
+- lua_func:
+    name: function returning explicit nil should succeed
+    key: $(test)_PASS
+    file: $(test_path)$(psep)lua_func.lua
+    func_name: return_explicit_nil
+
 - group:
     name: context_id tests
     steps:

test/validation/items/py_func/py_func.py

@@ -54,3 +54,10 @@ def test_delgd():
     tm.delgd("_py_delgd_test")
     assert tm.gd("_py_delgd_test", None) is None
     return 0
+
+def return_nothing():
+    # Falls off the end: implicit None return, no error.
+    pass
+
+def return_explicit_none():
+    return None

test/validation/items/py_func/test.tum

@@ -196,6 +196,18 @@
     file: $(test_path)$(psep)py_func.py
     func_name: test_delgd
 
+- py_func:
+    name: function returning nothing should succeed
+    key: $(test)_PASS
+    file: $(test_path)$(psep)py_func.py
+    func_name: return_nothing
+
+- py_func:
+    name: function returning explicit None should succeed
+    key: $(test)_PASS
+    file: $(test_path)$(psep)py_func.py
+    func_name: return_explicit_none
+
 - group:
     name: context_id tests
     steps: