Fixes that the settings system crashes on an empty or missing settings file

Author: pdelvo

StyleCop.Analyzers/StyleCop.Analyzers.Test/Settings/SettingsUnitTests.cs

@@ -152,6 +152,19 @@ public async Task VerifyInvalidJsonBehaviorAsync()
             Assert.Equal("Copyright (c) PlaceholderCompany. All rights reserved.", styleCopSettings.DocumentationRules.GetCopyrightText("unused"));
         }
 
+        [Fact]
+        public async Task VerifyEmptyOrMissingFileAsync()
+        {
+            var settings = string.Empty;
+            var context = await CreateAnalysisContextAsync(settings).ConfigureAwait(false);
+
+            var styleCopSettings = context.GetStyleCopSettings(CancellationToken.None);
+
+            // The result is the same as the default settings.
+            Assert.Equal("PlaceholderCompany", styleCopSettings.DocumentationRules.CompanyName);
+            Assert.Equal("Copyright (c) PlaceholderCompany. All rights reserved.", styleCopSettings.DocumentationRules.GetCopyrightText("unused"));
+        }
+
         private static async Task<SyntaxTreeAnalysisContext> CreateAnalysisContextAsync(string stylecopJSON)
         {
             var projectId = ProjectId.CreateNewId();

StyleCop.Analyzers/StyleCop.Analyzers/Settings/SettingsHelper.cs

@@ -127,6 +127,12 @@ private static StyleCopSettings GetStyleCopSettings(SourceText text, Deserializa
             try
             {
                 var root = JsonConvert.DeserializeObject<SettingsFile>(text.ToString());
+
+                if (root == null)
+                {
+                    throw new JsonException($"Settings file was missing or empty.");
+                }
+
                 return root.Settings;
             }
             catch (JsonException) when (failureBehavior == DeserializationFailureBehavior.ReturnDefaultSettings)
@@ -160,6 +166,12 @@ private static StyleCopSettings GetStyleCopSettings(ImmutableArray<AdditionalTex
                     {
                         SourceText additionalTextContent = additionalFile.GetText(cancellationToken);
                         var root = JsonConvert.DeserializeObject<SettingsFile>(additionalTextContent.ToString());
+
+                        if (root == null)
+                        {
+                            throw new JsonException($"Settings file at `{Path.GetFileName(additionalFile.Path)}` was missing or empty.");
+                        }
+
                         return root.Settings;
                     }
                 }