[System.Data] Call SqlConnection.ClearAllPools() only when we had a connection

When ConnectionManager.Instance.Sql.ConnectionString throws in
SetUp() we'd still call SqlConnection.ClearAllPools() in the TearDown()
which happens for example when the connection string environment
variable is not set.

This is a problem on WatchOS since SqlConnection.ClearAllPools()
throws PlatformNotSupportedException there, making the tests fail.
Instead we now check whether the connectionString is set and only
call the method in that case.

mcs/class/System.Data/Test/ProviderTests/System.Data.SqlClient/SqlConnectionTest.cs

@@ -60,7 +60,8 @@ namespace MonoTests.System.Data.Connected.SqlClient
 		{
 			if (conn != null)
 				conn.Dispose ();
-			SqlConnection.ClearAllPools ();
+			if (connectionString != null)
+				SqlConnection.ClearAllPools ();
 		}
 
 		[Test]