Disable SA0002 (Invalid settings file) for empty projects

Fixes #3453

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test/SpecialRules/SA0002UnitTests.cs

@@ -34,6 +34,30 @@ public async Task TestMissingSettingsAsync()
             await VerifyCSharpDiagnosticAsync(TestCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
         }
 
+        [Fact]
+        [WorkItem(3453, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3453")]
+        public async Task TestNoSourceFilesAsync()
+        {
+            string emptySettings = @"{ ""settings"": { } }";
+            await new CSharpTest
+            {
+                TestState =
+                {
+                    Sources = { string.Empty },
+                    AdditionalFiles = { ("stylecop.json", emptySettings) },
+                    AdditionalProjects =
+                    {
+                        ["EmptyProjectWithSettings"] =
+                        {
+                            // The main test state doesn't allow empty Sources, so we use a second project to validate
+                            // the completely empty case.
+                            AdditionalFiles = { ("stylecop.json", emptySettings) },
+                        },
+                    },
+                },
+            }.RunAsync(CancellationToken.None).ConfigureAwait(false);
+        }
+
         [Fact]
         public async Task TestValidSettingsAsync()
         {

StyleCop.Analyzers/StyleCop.Analyzers/SpecialRules/SA0002InvalidSettingsFile.cs

@@ -51,9 +51,15 @@ public override void Initialize(AnalysisContext context)
 
         private static void HandleCompilation(CompilationAnalysisContext context)
         {
+            var firstSyntaxTree = context.Compilation.SyntaxTrees.FirstOrDefault();
+            if (firstSyntaxTree is null)
+            {
+                return;
+            }
+
             try
             {
-                SettingsHelper.GetStyleCopSettings(context.Options, context.Compilation?.SyntaxTrees.FirstOrDefault(), DeserializationFailureBehavior.ThrowException, context.CancellationToken);
+                SettingsHelper.GetStyleCopSettings(context.Options, firstSyntaxTree, DeserializationFailureBehavior.ThrowException, context.CancellationToken);
             }
             catch (Exception ex) when (ex is JsonParseException || ex is InvalidSettingsException)
             {