Fix ArgumentNullException in the ExposeAllPorts method

Also add a test that covers the `ExposeAllPorts` method and two tests that cover the mutual exclusivity between `ExposePort` and `ExposeAllPorts` methods.

Author: 0xced

Ductus.FluentDocker.Tests/FluentApiTests/FluentContainerBasicTests.cs

@@ -191,6 +191,40 @@ public void ImplicitPortMappingShouldWork()
       }
     }
 
+    [TestMethod]
+    [TestCategory("CI")]
+    public void FullImplicitPortMappingShouldWork()
+    {
+      using (
+        var container =
+          Fd.UseContainer()
+            .UseImage("postgres:9.6-alpine")
+            .ExposeAllPorts()
+            .WithEnvironment("POSTGRES_PASSWORD=mysecretpassword")
+            .Build()
+            .Start())
+      {
+        var endpoint = container.ToHostExposedEndpoint("5432/tcp");
+        AreNotEqual(0, endpoint.Port);
+      }
+    }
+
+    [TestMethod]
+    [TestCategory("CI")]
+    public void ExposeAllPortsIsMutuallyExclusiveWithExposePort()
+    {
+      var exception = ThrowsException<FluentDockerNotSupportedException>(() => Fd.UseContainer().ExposePort(5432).ExposeAllPorts());
+      AreEqual("ExposeAllPorts is mutually exclusive with ExposePort methods. Do not call ExposePort if you want to expose all ports.", exception.Message);
+    }
+
+    [TestMethod]
+    [TestCategory("CI")]
+    public void ExposePortIsMutuallyExclusiveWithExposeAllPorts()
+    {
+      var exception = ThrowsException<FluentDockerNotSupportedException>(() => Fd.UseContainer().ExposeAllPorts().ExposePort(5432));
+      AreEqual("ExposePort is mutually exclusive with ExposeAllPorts methods. Do not call ExposeAllPorts if you want to explicitly expose ports.", exception.Message);
+    }
+
     [TestMethod]
     [TestCategory("CI")]
     public void WaitForPortShallWork()

Ductus.FluentDocker/Builders/ContainerBuilder.cs

@@ -358,7 +358,7 @@ private void EnsurePublishAllPortsIsFalse()
 
     private void EnsurePortMappingsIsEmpty()
     {
-      if (_config.CreateParams.PortMappings.Any())
+      if (_config.CreateParams.PortMappings?.Any() == true)
       {
         throw new FluentDockerNotSupportedException($"{nameof(ExposeAllPorts)} is mutually exclusive with {nameof(ExposePort)} methods. " +
                                                     $"Do not call {nameof(ExposePort)} if you want to expose all ports.");