Fix ArgumentNullException when provider is unable to open a connection (nhibernate#2882)

Back-port of nhibernate#2794

Author: hazzik

src/NHibernate/Async/Tool/hbm2ddl/SchemaExport.cs

@@ -525,7 +525,11 @@ public async Task ExecuteAsync(Action<string> scriptAction, bool execute, bool j
 			{
 				if (connectionProvider != null)
 				{
-					connectionProvider.CloseConnection(connection);
+					if (connection != null)
+					{
+						connectionProvider.CloseConnection(connection);
+					}
+
 					connectionProvider.Dispose();
 				}
 			}

src/NHibernate/Tool/hbm2ddl/SchemaExport.cs

@@ -468,7 +468,11 @@ private void InitConnectionAndExecute(Action<string> scriptAction, bool execute,
 			{
 				if (connectionProvider != null)
 				{
-					connectionProvider.CloseConnection(connection);
+					if (connection != null)
+					{
+						connectionProvider.CloseConnection(connection);
+					}
+
 					connectionProvider.Dispose();
 				}
 			}

teamcity.build

@@ -207,7 +207,7 @@
 		<property name="db-service" value="MySQL57" />
 		<property name="nhibernate.connection.driver_class" value="NHibernate.Driver.MySqlDataDriver" />
 		<property name="nhibernate.dialect"	value="NHibernate.Dialect.MySQL5Dialect" />
-		<property name="nhibernate.connection.connection_string" value="Data Source=localhost;Database=nhibernate;User ID=nhibernate;Password=nhibernate;Protocol=memory;Old Guids=True;" />
+		<property name="nhibernate.connection.connection_string" value="Data Source=localhost;Database=nhibernate;User ID=nhibernate;Password=nhibernate;Protocol=memory;Old Guids=True;SslMode=none;" />
 	</target>
 
 	<target name="setup-teamcity-sqlanywhere">