(CAT-2164) Correct yaml_syntax_validator to error on empty file

Previously empty yaml files would pass and be accepted as valid when PDK validate was run.
However this was contrasted by Puppet and the puppetserver requiring files to always contain a valid yaml and so we are updating the pdk to be in line with this.

lib/pdk/validate/yaml/yaml_syntax_validator.rb

@@ -49,15 +49,25 @@ def validate_target(report, target)
           end
 
           begin
-            ::YAML.safe_load(PDK::Util::Filesystem.read_file(target), permitted_classes: YAML_ALLOWLISTED_CLASSES, permitted_symbols: [], aliases: true)
-
-            report.add_event(
-              file: target,
-              source: name,
-              state: :passed,
-              severity: 'ok'
-            )
-            0
+            data = ::YAML.safe_load(PDK::Util::Filesystem.read_file(target), permitted_classes: YAML_ALLOWLISTED_CLASSES, permitted_symbols: [], aliases: true)
+            if data.is_a?(Hash)
+              report.add_event(
+                file: target,
+                source: name,
+                state: :passed,
+                severity: 'ok'
+              )
+              0
+            else
+              report.add_event(
+                file: target,
+                source: name,
+                state: :failure,
+                severity: 'error',
+                message: format('File does not contain a valid YAML hash.')
+              )
+              1
+            end
           rescue Psych::SyntaxError => e
             report.add_event(
               file: target,

spec/unit/pdk/validate/yaml/yaml_syntax_validator_spec.rb

@@ -109,5 +109,20 @@
         expect(return_value).to eq(1)
       end
     end
+
+    context 'when a target is provided that contains no YAML' do
+      let(:target) { { name: '.sync.yaml', content: '' } }
+
+      it 'adds a failure event to the report' do
+        expect(report).to receive(:add_event).with({
+                                                     file: target[:name],
+                                                     source: 'yaml-syntax',
+                                                     state: :failure,
+                                                     severity: 'error',
+                                                     message: a_string_matching(/\AFile does not contain a valid YAML hash/)
+                                                   })
+        expect(return_value).to eq(1)
+      end
+    end
   end
 end