Allow blank password with username option (resolves Topshelf#285);

  * Whilst ensuring specified when username is.

src/Topshelf.Tests/CommandLine_Specs.cs

@@ -320,6 +320,34 @@ public void Extensible_the_command_line_should_be_yet_again()
             Assert.AreEqual("11", volumeLevel);
         }
 
+        [Test]
+        public void Should_require_password_option_when_specifying_username()
+        {
+            Assert.Throws<Topshelf.HostConfigurationException>(() =>
+            {
+                Host host = HostFactory.New(x =>
+                {
+                    x.Service<MyService>();
+                    x.ApplyCommandLine("install -username \"Joe\"");
+                });
+            });
+        }
+
+        [Test]
+        public void Will_allow_blank_password_when_specifying_username()
+        {
+            Host host = HostFactory.New(x =>
+                {
+                    x.Service<MyService>();
+                    x.ApplyCommandLine("install -username \"Joe\" -password \"\"");
+                });
+
+            Assert.IsInstanceOf<InstallHost>(host);
+            var installHost = (InstallHost)host;
+            Assert.AreEqual("Joe", installHost.InstallSettings.Username);
+            Assert.AreEqual("", installHost.InstallSettings.Password);
+        }
+
         class MyService : ServiceControl
         {
             public bool Start(HostControl hostControl)

src/Topshelf/Configuration/HostConfigurators/RunAsUserHostConfigurator.cs

@@ -45,7 +45,7 @@ public IEnumerable<ValidateResult> Validate()
         {
             if (string.IsNullOrEmpty(Username))
                 yield return this.Failure("Username", "must be specified for a User account type");
-            if (string.IsNullOrEmpty(Password))
+            if (Password == null)
                 yield return this.Failure("Password", "must be specified for a User account type");
         }
     }