scripts: Don't generate functions for empty bit flags

Bit flags that don't have any members (e.g. VkRenderPassCreateFlagBits)
don't generate in the vulkan headers, but were generating for the enum
string helper - causing compile errors.
Added a check for such types with no flags to fix the error.
diff --git a/scripts/helper_file_generator.py b/scripts/helper_file_generator.py
index b48c42f..12084e9 100644
--- a/scripts/helper_file_generator.py
+++ b/scripts/helper_file_generator.py
@@ -194,7 +194,8 @@
             for elem in groupElem.findall('enum'):
                 if elem.get('supported') != 'disabled' and elem.get('alias') is None:
                     value_set.add(elem.get('name'))
-            self.enum_output += self.GenerateEnumStringConversion(groupName, value_set)
+            if value_set != set():
+                self.enum_output += self.GenerateEnumStringConversion(groupName, value_set)
         elif self.helper_file_type == 'object_types_header':
             if groupName == 'VkDebugReportObjectTypeEXT':
                 for elem in groupElem.findall('enum'):