org.pentaho.di.core.exception.KettleDatabaseException: Error occured while trying to connect to the database Error connecting to database: (using class org.gjt.mm.mysql.Driver) java.lang.NullPointerException

This kind of error happens when the driver is not the correct one, just change it!

because maybe you are using: mysql-connector-java-5.1.34.jar, or mysql-connector-java-5.0.8.tar..which uses:   org.gjt.mm.mysql.Driver para invocar el driver
but, both are incorrect for Pentaho spoon

The correct is: jdbc_mysql.jar  because it uses the driver :  com.mysql.jdbc.Driver