Fixed the last change not to crash with "internal error in WriteCurlrc"

when installing with a proxy (boo#929868).

In 7302ee6 the change from Ops.add to +
broke it because I did not think of getting an explicit nil via Import:

InstallInfConvertor#write_proxy makes ProxyClass#user nil:
https://github.com/yast/yast-network/blob/5595e2ece959cdadf144e095d1b0e0d1bb7059e2/src/lib/network/install_inf_convertor.rb#L251

src/modules/Proxy.rb

@@ -136,9 +136,9 @@ def EscapeForCurlrc(s)
 
     def WriteCurlrc
       proxyuser = nil
-      if @user != ""
+      if @user && !@user.empty?
         proxyuser = @user
-        proxyuser = @user + ":" + @pass if @pass != ""
+        proxyuser << ":" << @pass if @pass && !@pass.empty?
       end
 
       options = {

test/write_test.rb

@@ -48,6 +48,35 @@
       expect(subject.WriteCurlrc).to be true
     end
 
+    it "writes proxy settings, and a comment, when proxy is enabled via InstallInfConvertor" do
+      subject.Import({ "enabled"        => true,
+                       "http_proxy"     => "proxy.example.org:3128",
+                       "proxy_user"     => nil,
+                       "proxy_password" => nil })
+
+      expect(Yast::SCR).to receive(:Write).
+        with(path_matching(/^\.root\.curlrc\..*\."comment"/), /Changed/).
+        once.and_return true
+
+      expect(Yast::SCR).to receive(:Write).
+        with(path(".root.curlrc.\"--proxy\""), "proxy.example.org:3128").
+        once.and_return true
+
+      expect(Yast::SCR).to receive(:Write).
+        with(path(".root.curlrc.\"--noproxy\""), "localhost").
+        once.and_return true
+
+      allow(Yast::SCR).to receive(:Write).
+        with(path_matching(/^\.root\.curlrc/), nil).
+        and_return true
+
+      expect(Yast::SCR).to receive(:Write).
+        with(path(".root.curlrc"), nil).
+        once.and_return true
+
+      expect(subject.WriteCurlrc).to be true
+    end
+
     it "writes a no-proxy setting" do
       subject.Import({ "enabled"    => true,
                        "http_proxy" => "proxy.example.org:3128",