[rubygems/rubygems] Return early if filename is empty

https://github.com/rubygems/rubygems/commit/1b9ab33083

lib/rubygems/config_file.rb

@@ -334,7 +334,7 @@ if you believe they were disclosed to a third party.
     yaml_errors = [ArgumentError]
     yaml_errors << Psych::SyntaxError if defined?(Psych::SyntaxError)
 
-    return {} unless filename and File.exist? filename
+    return {} unless filename && !filename.empty? && File.exist?(filename)
 
     begin
       content = Gem::SafeYAML.load(File.read(filename))

test/rubygems/test_gem_config_file.rb

@@ -19,7 +19,7 @@ class TestGemConfigFile < Gem::TestCase
     Gem::ConfigFile::PLATFORM_DEFAULTS.clear
 
     @env_gemrc = ENV['GEMRC']
-    ENV['GEMRC'] = nil
+    ENV['GEMRC'] = ''
 
     util_config_file
   end