Allow empty string value for config entries

PostgreSQL supports and allows config entries, such as those in
postgresql.conf, to be set to the empty string. The
`postgresql::server::config_entry` defined type, however, requires
String[1] when supplying string values. This doesn't allow for the empty
string.

This change relaxes the allowed data types for the `value` parameter of
`postgresql::server::config_entry` to `String` from `String[1]` and adds
a spec test to support the change.

Fixes #1602

manifests/server/config_entry.pp

@@ -10,7 +10,7 @@
 define postgresql::server::config_entry (
   Enum['present', 'absent']                               $ensure  = 'present',
   String[1]                                               $key     = $name,
-  Optional[Variant[String[1], Numeric, Array[String[1]]]] $value   = undef,
+  Optional[Variant[String, Numeric, Array[String[1]]]]    $value   = undef,
   Stdlib::Absolutepath                                    $path    = $postgresql::server::postgresql_conf_path,
   Optional[String[1]]                                     $comment = undef,
   String[1]                                               $instance_name = 'main',

spec/defines/server/config_entry_spec.rb

@@ -79,4 +79,19 @@
         .that_notifies('Postgresql::Server::Instance::Service[main]')
     end
   end
+
+  # Example use case for empty values: using transaction-bound session
+  # variables, like `SET LOCAL awesome_app.tenant_id = 92834` requires the
+  # variable to be defined in postgresql.conf and initialised with a default
+  # value (at least for PostgreSQL 13 and older, possibly also in newer
+  # versions). The default value can, and sometimes, depending on the
+  # application, should, be empty.
+  context 'set a config entry value to the empty string' do
+    let(:params) { { ensure: 'present', name: 'mydatabase.app_specific_parameter', value: '' } }
+
+    it 'sets value to the empty string' do
+      expect(subject).to contain_postgresql_conf('mydatabase.app_specific_parameter').with(name: 'mydatabase.app_specific_parameter',
+                                                                                           value: '')
+    end
+  end
 end