add test for old behaviour where kernel_name="" uses file specified

In order to not use the default "python" behvaiour, I set the kernel to 
be "python3". However, that means this can only work if we have a 
python3 kernel available.

nbconvert/preprocessors/tests/test_execute.py

@@ -21,6 +21,8 @@
 from .base import PreprocessorTestsBase
 from ..execute import ExecutePreprocessor, CellExecutionError, executenb, find_kernel_name
 
+from jupyter_client.kernelspec import KernelSpecManager
+
 from nbconvert.filters import strip_ansi
 from testpath import modified_env
 from ipython_genutils.py3compat import string_types
@@ -151,6 +153,19 @@ def test_empty_path(self):
         res['metadata']['path'] = ''
         input_nb, output_nb = self.run_notebook(filename, {}, res)
         self.assert_notebooks_equal(input_nb, output_nb)
+
+    @pytest.mark.xfail("python3" not in KernelSpecManager().find_kernel_specs(),
+                        reason="requires a python3 kernelspec")
+    def test_empty_kernel_name(self):
+        """Can kernel in nb metadata be found when an empty string is passed?
+        
+        Note: this pattern should be discouraged in practice.
+        Passing in no kernel_name to ExecutePreprocessor is preferable.
+        """
+        filename = os.path.join(current_dir, 'files', 'UnicodePy3.ipynb')
+        res = self.build_resources()
+        input_nb, output_nb = self.run_notebook(filename, {"kernel_name": ""}, res)
+        self.assert_notebooks_equal(input_nb, output_nb)
 
     def test_disable_stdin(self):
         """Test disabling standard input"""