don't try template names we know to be invalid

if template_file is empty, only one choice makes sense: module name

IPython/nbconvert/exporters/exporter.py

@@ -215,10 +215,14 b' class Exporter(LoggingConfigurable):'
         # as if the name is explicitly specified, then try the name as a 
         # 'flavor', and lastly just try to load the template by module name.
         module_name = self.__module__.rsplit('.', 1)[-1]
-        try_names = [self.template_file + self.template_extension,
-                     self.template_file,
-                     module_name + '_' + self.template_file + self.template_extension,
-                     module_name + self.template_extension]
+        try_names = []
+        if self.template_file:
+            try_names.extend([
+                self.template_file + self.template_extension,
+                self.template_file,
+                module_name + '_' + self.template_file + self.template_extension,
+            ])
+        try_names.append(module_name + self.template_extension)
         for try_name in try_names:
             self.log.debug("Attempting to load template %s", try_name)
             try: