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

[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.