[ruby/rexml] xpath local_name: fix a bug that nil is returned for nonexistent case

It must be an empty string.

https://github.com/ruby/rexml/commit/81bc7cd4f5

lib/rexml/functions.rb

@@ -67,10 +67,11 @@ module REXML
     end
 
     # UNTESTED
-    def Functions::local_name( node_set=nil )
-      get_namespace( node_set ) do |node|
+    def Functions::local_name(node_set=nil)
+      get_namespace(node_set) do |node|
         return node.local_name
       end
+      ""
     end
 
     def Functions::namespace_uri( node_set=nil )

test/rexml/functions/test_base.rb

@@ -3,6 +3,7 @@ require "test/unit/testcase"
 
 require "rexml/document"
 
+# TODO: Split me
 module REXMLTests
   class FunctionsTester < Test::Unit::TestCase
     include REXML

test/rexml/functions/test_local_name.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: false
+
+require "test/unit"
+require "rexml/document"
+require "rexml/functions"
+
+module REXMLTests
+  class TestFunctionsLocalName < Test::Unit::TestCase
+    def setup
+      REXML::Functions.context = nil
+    end
+
+    def test_one
+      document = REXML::Document.new(<<-XML)
+<root xmlns:x="http://example.com/x/">
+  <x:child/>
+</root>
+      XML
+      node_set = document.root.children
+      assert_equal("child", REXML::Functions.local_name(node_set))
+    end
+
+    def test_multiple
+      document = REXML::Document.new(<<-XML)
+<root xmlns:x="http://example.com/x/">
+  <x:child1/>
+  <x:child2/>
+</root>
+      XML
+      node_set = document.root.children
+      assert_equal("child1", REXML::Functions.local_name(node_set))
+    end
+
+    def test_nonexistent
+      assert_equal("", REXML::Functions.local_name([]))
+    end
+
+    def test_context
+      document = REXML::Document.new("<root/>")
+      REXML::Functions.context = {node: document.root}
+      assert_equal("root", REXML::Functions.local_name())
+    end
+  end
+end