treat returning None from write_<param> properly

as intended originally, returning None should be the same
as returning the new value from the argument

Change-Id: I5596d2b6f6f525efd4dc410bbe22dad6f1fb2017
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30659
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_modules.py

@@ -727,3 +727,15 @@ def test_super_call():
 
     assert type(m).__name__ == '_Mod'
     assert type(m).__mro__[1:5] == (Mod, Base, Readable, Module)
+
+
+def test_write_method_returns_none():
+    class Mod(Module):
+        a = Parameter('', FloatRange(), readonly=False)
+
+        def write_a(self, value):
+            return None
+
+    mod = Mod('mod', LoggerStub(), {'description': ''}, ServerStub({}))
+    mod.write_a(1.5)
+    assert mod.a == 1.5