fix \NULL with following space and no {} (two instances)

Doc/ext/newtypes.tex

@@ -460,8 +460,8 @@ restrict the types of objects that can be assigned to the Python
 attributes.  We expect the first and last names to be strings, but any
 Python objects can be assigned.  Further, the attributes can be
 deleted, setting the C pointers to \NULL.  Even though we can make
-sure the members are initialized to non-\NULL values, the members can
-be set to \NULL if the attributes are deleted.
+sure the members are initialized to non-\NULL{} values, the members can
+be set to \NULL{} if the attributes are deleted.
 
 We define a single method, \method{name}, that outputs the objects
 name as the concatenation of the first and last names.