Fix user qualification tool runtime error in get_platform_name for onprem platform

[cindyyuanjiang]

Fixes #683

Changes
We fix a bug in function get_platform_name definition for onprem platform. The original implementation could lead to referencing an unassigned variable.

Testing
For user qualification tool, we tested the following commands:

spark_rapids_user_tools onprem qualification --eventlogs <my-event-logs>
spark_rapids_user_tools onprem qualification --eventlogs <my-event-logs> --cpu_cluster <my-cpu-cluster-file>
spark_rapids_user_tools onprem qualification --eventlogs <my-event-logs> --cpu_cluster <my-cpu-cluster-file> --target_platform dataproc

For user profiling tool:

spark_rapids_user_tools onprem profiling --eventlogs <my-event-logs>

[parthosa]

Thanks @cindyyuanjiang

[nartal1]

Thanks @cindyyuanjiang !