ConnectionManager no longer uses the underlying XADataSource's credentials when none are provided in its Properties

I believe that this commit has introduced a regression in how ConnectionManager handles credentials. Previously, if the Properties contained an XADataSource and no credentials, the data source's default credentials would be used, i.e. XADataSource.getXAConnection() would be called by ProvidedXADataSourceConnection. With the aforelinked change, ConnectionManager now coerces null credentials to empty strings. As a result XADataSource.getXAConnection(String, String) is called by ProvidedXADataSourceConnection with the empty credentials.