Clarify comments for xml_*::empty()

"empty" can be mistaken for "has no children" in case of xml_node in
particular; we now use both empty and null in the comment in hopes that
it may help.

src/pugixml.hpp

@@ -396,7 +396,7 @@ namespace pugi
  bool operator<=(const xml_attribute& r) const;
  bool operator>=(const xml_attribute& r) const;
 
- // Check if attribute is empty
+ // Check if attribute is empty (null)
  bool empty() const;
 
  // Get attribute name/value, or "" if attribute is empty
@@ -507,7 +507,7 @@ namespace pugi
  bool operator<=(const xml_node& r) const;
  bool operator>=(const xml_node& r) const;
 
- // Check if node is empty.
+ // Check if node is empty (null)
  bool empty() const;
 
  // Get node type
@@ -764,7 +764,7 @@ namespace pugi
  // Borland C++ workaround
  bool operator!() const;
 
- // Check if text object is empty
+ // Check if text object is empty (null)
  bool empty() const;
 
  // Get text, or "" if object is empty