tools: don't set empty attributes in "generate-docs-nm-property-infos.py"

If the information is missing, the entire attribute should not be there.
Don't set it to the empty word.

Also, don't alias the "variable" attribute to the "name". It's not clear
what the "variable" fields is supposed to mean, but if it's not
explicitly set, don't make up the information. If a user of that
information cares, the can always fallback to the "name".

tools/generate-docs-nm-property-infos.py

@@ -132,12 +132,7 @@ def write_data(tag, setting_node, line_no, parsed_data):
 
         v = parsed_data.get(k, None)
         if v is None:
-            if k == "variable":
-                v = name
-            elif k == 'description-docbook':
-                continue
-            else:
-                v = ""
+            continue
 
         if xmltype == KEYWORD_XML_TYPE_NESTED:
             # Set as XML nodes. The input data is XML itself.