Auto merge of rubygems#1605 - djberg96:cert_check, r=segiddins

Raise an explicit error if Signer#sign is called with no certs

# Description:

This PR addresses an unfriendly error message that I brought up in rubygems#1413. It now explicitly raises an error if no certs are found. I also added a test for it.
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lib/rubygems/security/signer.rb

@@ -102,6 +102,8 @@ def load_cert_chain # :nodoc:
   def sign data
     return unless @key
 
+    raise Gem::Security::Exception, 'no certs provided' if @cert_chain.empty?
+
     if @cert_chain.length == 1 and @cert_chain.last.not_after < Time.now then
       re_sign_key
     end

test/rubygems/test_gem_security_signer.rb

@@ -205,5 +205,13 @@ def test_sign_wrong_key
     end
   end
 
+  def test_sign_no_certs
+    signer = Gem::Security::Signer.new ALTERNATE_KEY, []
+
+    assert_raises Gem::Security::Exception do
+      signer.sign 'hello'
+    end
+  end
+
 end if defined?(OpenSSL::SSL)