Fixed a crasher in delegates pass when processing libraries with no declarations.

This is admittedly a pretty unusual case but still a valid one.

Also adds a new test to make sure we do not regress.

src/Generator/Passes/DelegatesPass.cs

@@ -40,8 +40,12 @@ namespace CppSharp.Passes
             foreach (var library in Driver.Options.Libraries.Where(l => !libsDelegates.ContainsKey(l)))
                 libsDelegates.Add(library, new Dictionary<string, DelegateDefinition>());
 
-            var unit = context.TranslationUnits.Last(u => u.IsValid && u.IsGenerated &&
+            var unit = context.TranslationUnits.LastOrDefault(u => u.IsValid && u.IsGenerated &&
                 !u.IsSystemHeader && u.HasDeclarations);
+
+            if (unit == null)
+                return false;
+
             namespaceDelegates = new Namespace { Name = DelegatesNamespace, Namespace = unit };
 
             var result = base.VisitLibrary(context);

tests/Empty/Empty.Tests.cs

@@ -0,0 +1,10 @@
+using System;
+using EmptyTest;
+using CppSharp.Utils;
+using NUnit.Framework;
+
+[TestFixture]
+public class EmptyTests : GeneratorTestFixture
+{
+}
+ 

tests/Empty/Empty.cpp

@@ -0,0 +1 @@
+#include "Empty.h"

tests/Empty/Empty.cs

@@ -0,0 +1,34 @@
+using CppSharp.AST;
+using CppSharp.Generators;
+using CppSharp.Utils;
+
+namespace CppSharp.Tests
+{
+    public class EmptyTestsGenerator : GeneratorTest
+    {
+        public EmptyTestsGenerator(GeneratorKind kind)
+            : base("Empty", kind)
+        {
+        }
+
+        public override void Setup(Driver driver)
+        {
+            base.Setup(driver);
+            driver.Options.OutputNamespace = "EmptyTest";
+        }
+
+        public override void SetupPasses(Driver driver)
+        {
+        }
+
+        public override void Preprocess(Driver driver, ASTContext ctx)
+        {
+        }
+
+        public static void Main(string[] args)
+        {
+            ConsoleDriver.Run(new EmptyTestsGenerator(GeneratorKind.CSharp));
+            ConsoleDriver.Run(new EmptyTestsGenerator(GeneratorKind.CLI));
+        }
+    }
+}

tests/Empty/Empty.h

@@ -0,0 +1,2 @@
+#include "../Tests.h"
+

tests/Empty/premake4.lua

@@ -0,0 +1,2 @@
+group "Tests/Empty"
+  SetupTestProject("Empty")