Node: do not add "::()" to nodeid

Fixes https://github.com/pytest-dev/pytest/issues/4127.

src/_pytest/nodes.py

@@ -27,7 +27,7 @@ def _splitnode(nodeid):
         ''
         'testing/code'
         'testing/code/test_excinfo.py'
-        'testing/code/test_excinfo.py::TestFormattedExcinfo::()'
+        'testing/code/test_excinfo.py::TestFormattedExcinfo'
 
     Return values are lists e.g.
         []
@@ -39,7 +39,7 @@ def _splitnode(nodeid):
         # If there is no root node at all, return an empty list so the caller's logic can remain sane
         return []
     parts = nodeid.split(SEP)
-    # Replace single last element 'test_foo.py::Bar::()' with multiple elements 'test_foo.py', 'Bar', '()'
+    # Replace single last element 'test_foo.py::Bar' with multiple elements 'test_foo.py', 'Bar'
     parts[-1:] = parts[-1].split("::")
     return parts
 
@@ -47,7 +47,7 @@ def _splitnode(nodeid):
 def ischildnode(baseid, nodeid):
     """Return True if the nodeid is a child node of the baseid.
 
-    E.g. 'foo/bar::Baz::()' is a child of 'foo', 'foo/bar' and 'foo/bar::Baz', but not of 'foo/blorp'
+    E.g. 'foo/bar::Baz' is a child of 'foo', 'foo/bar' and 'foo/bar::Baz', but not of 'foo/blorp'
     """
     base_parts = _splitnode(baseid)
     node_parts = _splitnode(nodeid)
@@ -107,10 +107,12 @@ class Node(object):
         self._name2pseudofixturedef = {}
 
         if nodeid is not None:
+            assert "::()" not in nodeid
             self._nodeid = nodeid
         else:
-            assert parent is not None
-            self._nodeid = self.parent.nodeid + "::" + self.name
+            self._nodeid = self.parent.nodeid
+            if self.name != "()":
+                self._nodeid += "::" + self.name
 
     @property
     def ihook(self):