(MODULES-10246) Proxy settings allow empty string

This PR enables the Yum Repo Core to accept an empty string as proxy
setting, to bypass any global proxy settings when accessing the
repository.

In case of EL 6/7, backwards compatibility was achived by replacing
empty string with _none_.

lib/puppet/type/yumrepo.rb

@@ -304,19 +304,26 @@
 
   newproperty(:proxy) do
     desc "URL of a proxy server that Yum should use when accessing this repository.
-      This attribute can also be set to `'_none_'`, which will make Yum bypass any
+      This attribute can also be set to `'_none_'` or `''`(empty string), which will make Yum bypass any
       global proxy settings when accessing this repository.
       #{ABSENT_DOC}"
 
     newvalues(%r{.*}, :absent)
     validate do |value|
       next if value.to_s =~ %r{^(absent|_none_)$}
-      parsed = URI.parse(value)
+      next if value.to_s.empty?
 
+      parsed = URI.parse(value)
       unless VALID_SCHEMES.include?(parsed.scheme)
         raise _('Must be a valid URL')
       end
     end
+
+    # Munges empty string (option available starting with EL 8)
+    # Using '_none_' as backwards compatibility
+    munge do |value|
+      value.to_s.empty? ? '_none_' : value.to_s
+    end
   end
 
   newproperty(:proxy_username) do

spec/unit/type/yumrepo_spec.rb

@@ -312,13 +312,30 @@
 
     describe 'proxy' do
       it_behaves_like 'a yumrepo parameter that can be absent', :proxy
+      it_behaves_like 'a yumrepo parameter that accepts a single URL', :proxy
+
       it 'accepts _none_' do
         described_class.new(
           name: 'puppetlabs',
           proxy: '_none_',
         )
       end
-      it_behaves_like 'a yumrepo parameter that accepts a single URL', :proxy
+
+      it 'accepts an empty string' do
+        described_class.new(
+          name: 'puppetlabs',
+          proxy: '',
+        )
+      end
+
+      it 'munges empty string to \'_none_\' due to backwards compatibility' do
+        instance = described_class.new(name: 'puppetlabs', proxy: '')
+        expect(instance[:proxy]).to eq '_none_'
+      end
+
+      it 'does not raise any errors' do
+        expect { described_class.new(name: 'puppetlabs', proxy: '') }.not_to raise_error
+      end
     end
 
     describe 'proxy_username' do