ERP5 Python Scripts: fix setParameterSignature('')

programmatically calling script.setParameterSignature('') was
setting _params to None and making it causing errors when calling
the script

product/ERP5/Document/PythonScript.py

@@ -114,10 +114,10 @@ class PythonScript(XMLObject, ZopePythonScript, ExpressionMixin('expression')):
       override to call ZopePythonScript methods to force compiling code
       and prevent setting to None
       """
-      if value is None:
-        self._params = ''
-      else:
+      if value:
         self._baseSetParameterSignature(value)
+      else:
+        self._params = ''
       self._compile()
 
     def _setProxyRoleList(self, value):

product/ERP5/tests/testERP5Callable.py

@@ -55,6 +55,19 @@ class TestERP5PythonScript(ERP5TypeTestCase):
     self.portal.portal_skins.custom.manage_delObjects([self.id()])
     self.tic()
 
+  def test_default_params(self):
+    self.assertEqual(self.script.getParameterSignature(), '')
+
+  def test_set_params(self):
+    self.script.setParameterSignature('foo')
+    self.assertEqual(self.script.getParameterSignature(), 'foo')
+    self.script.setParameterSignature('')
+    self.assertEqual(self.script.getParameterSignature(), '')
+    self.script.setParameterSignature('bar')
+    self.assertEqual(self.script.getParameterSignature(), 'bar')
+    self.script.setParameterSignature(None) # Base_edit calls with None
+    self.assertEqual(self.script.getParameterSignature(), '')
+
   def test_manage_addPythonScriptThroughZMI(self):
     resp = self.publish(
       '/{}/portal_skins/manage_addProduct/ERP5/addPythonScriptThroughZMIForm'.format(self.portal.getId()),