Fix wrap_function on empty arguments

Caused by an implicit boolean check which should have been an explicit None check

PiperOrigin-RevId: 225096833

tensorflow/python/eager/function.py

@@ -340,7 +340,7 @@ class Function(object):
       TypeError: For invalid positional/keyword argument combinations.
     """
     if self._arg_keywords is None or self._num_positional_args is None:
-      if self._signature:
+      if self._signature is not None:
         if kwargs:
           raise NotImplementedError(
               "Keyword arguments not supported when calling a "

tensorflow/python/eager/wrap_function_test.py

@@ -19,6 +19,7 @@ from __future__ import print_function
 
 
 from tensorflow.python.eager import wrap_function
+from tensorflow.python.framework import constant_op
 from tensorflow.python.framework import dtypes
 from tensorflow.python.framework import ops
 from tensorflow.python.framework import tensor_spec
@@ -70,6 +71,14 @@ class WrapFunctionTest(test.TestCase):
     f_pruned = f_wrapped.prune(x_in[0], [x_out[0]])
     self.assertAllEqual(f_pruned(ops.convert_to_tensor(2.0)), [4.0])
 
+  def testNoArguments(self):
+
+    def f():
+      return constant_op.constant(1.)
+
+    f_wrapped = wrap_function.wrap_function(f, [])
+    self.assertAllEqual(1.0, f_wrapped())
+
 
 if __name__ == '__main__':
   ops.enable_eager_execution()