Make hosts / plugins / types always optional

The previous configuration did not allowed to create a host with only
plugins or types which is quite common, just like the plugin allows to
not pass hosts, plugins or types.  For consistency, never require any
"child" Struct.

While here, extend and reorder the tests.

spec/classes/collectd_plugin_threshold_spec.rb

@@ -7,7 +7,7 @@
         facts
       end
 
-      let :params do
+      let :custom_params do
         {
           'types' => [
             {
@@ -77,6 +77,31 @@
               content: %r{LoadPlugin threshold}
             )
           end
+        end
+
+        context ':ensure => present and empty configurations' do
+          let :params do
+            {
+              'hosts' => [
+                {
+                  'name' => 'example.com',
+                }
+              ],
+              'plugins' => [
+                {
+                  'name' => 'interface',
+                }
+              ],
+            }
+          end
+
+          it { is_expected.to compile }
+        end
+
+        context ':ensure => present and custom parameters' do
+          let :params do
+            custom_params
+          end
 
           it { is_expected.to contain_file('threshold.load').with(content: %r{<Type "foo">}) }
           it { is_expected.to contain_file('threshold.load').with(content: %r{WarningMin 0\.0}) }

templates/plugin/threshold.conf.epp

@@ -1,11 +1,11 @@
 <Plugin "threshold">
-  <%- $collectd::plugin::threshold::hosts.each |$host| { -%>
+  <%- $collectd::plugin::threshold::hosts.lest || { [] }.each |$host| { -%>
 <%= collectd::indent(epp('collectd/plugin/threshold/host.epp', { host => $host })) -%>
   <%- } -%>
-  <%- $collectd::plugin::threshold::plugins.each |$plugin| { -%>
+  <%- $collectd::plugin::threshold::plugins.lest || { [] }.each |$plugin| { -%>
 <%= collectd::indent(epp('collectd/plugin/threshold/plugin.epp', { plugin => $plugin })) -%>
   <%- } -%>
-  <%- $collectd::plugin::threshold::types.each |$type| { -%>
+  <%- $collectd::plugin::threshold::types.lest || { [] }.each |$type| { -%>
 <%= collectd::indent(epp('collectd/plugin/threshold/type.epp', { type => $type })) -%>
   <%- } -%>
 </Plugin>

templates/plugin/threshold/host.epp

@@ -1,8 +1,8 @@
 <Host "<%= $host['name'] %>">
-  <%- $host['plugins'].each |$plugin| { -%>
+  <%- $host['plugins'].lest || { [] }.each |$plugin| { -%>
 <%= collectd::indent(epp('collectd/plugin/threshold/plugin.epp', { plugin => $plugin })) -%>
   <%- } -%>
-  <%- $host['types'].each |$type| { -%>
+  <%- $host['types'].lest || { [] }.each |$type| { -%>
 <%= collectd::indent(epp('collectd/plugin/threshold/type.epp', { type => $type })) -%>
   <%- } -%>
 </Host>

templates/plugin/threshold/plugin.epp

@@ -2,7 +2,7 @@
   <%- if $plugin['instance'] != undef { -%>
   Instance "<%= $plugin['instance'] %>"
   <%- } -%>
-  <%- $plugin['types'].each |$type| { -%>
+  <%- $plugin['types'].lest || { [] }.each |$type| { -%>
 <%= collectd::indent(epp('collectd/plugin/threshold/type.epp', { type => $type })) -%>
   <%- } -%>
 </Plugin>

types/threshold/host.pp

@@ -1,5 +1,5 @@
 type Collectd::Threshold::Host = Struct[{
   name    => String[1],
-  plugins => Array[Collectd::Threshold::Plugin],
-  types   => Array[Collectd::Threshold::Type],
+  plugins => Optional[Array[Collectd::Threshold::Plugin]],
+  types   => Optional[Array[Collectd::Threshold::Type]],
 }]

types/threshold/plugin.pp

@@ -1,5 +1,5 @@
 type Collectd::Threshold::Plugin = Struct[{
   name     => String[1],
   instance => Optional[String[1]],
-  types    => Array[Collectd::Threshold::Type],
+  types    => Optional[Array[Collectd::Threshold::Type]],
 }]