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