Use Safe Navigation (&.) from Ruby 2.3 (#265)

We can simplify our code by using `&.`.

Author: pboling

lib/rexml/attribute.rb

@@ -130,10 +130,7 @@ def to_string
     end
 
     def doctype
-      if @element
-        doc = @element.document
-        doc.doctype if doc
-      end
+      @element&.document&.doctype
     end
 
     # Returns the attribute value, with entities replaced

lib/rexml/child.rb

@@ -83,8 +83,7 @@ def previous_sibling=(other)
     # Returns:: the document this child belongs to, or nil if this child
     # belongs to no document
     def document
-      return parent.document unless parent.nil?
-      nil
+      parent&.document
     end
 
     # This doesn't yet handle encodings

lib/rexml/doctype.rb

@@ -171,15 +171,11 @@ def write( output, indent=0, transitive=false, ie_hack=false )
     end
 
     def context
-      if @parent
-        @parent.context
-      else
-        nil
-      end
+      @parent&.context
     end
 
     def entity( name )
-      @entities[name].unnormalized if @entities[name]
+      @entities[name]&.unnormalized
     end
 
     def add child
@@ -288,8 +284,7 @@ def initialize name, middle, pub, sys
     end
 
     def to_s
-      context = nil
-      context = parent.context if parent
+      context = parent&.context
       notation = "<!NOTATION #{@name}"
       reference_writer = ReferenceWriter.new(@middle, @public, @system, context)
       reference_writer.write(notation)

lib/rexml/element.rb

@@ -473,8 +473,7 @@ def root
     # Related: #root, #root_node.
     #
     def document
-      rt = root
-      rt.parent if rt
+      root&.parent
     end
 
     # :call-seq:

lib/rexml/text.rb

@@ -200,10 +200,7 @@ def <=>( other )
     end
 
     def doctype
-      if @parent
-        doc = @parent.document
-        doc.doctype if doc
-      end
+      @parent&.document&.doctype
     end
 
     REFERENCE = /#{Entity::REFERENCE}/