How to avoid JDBC connection password saving/visibility in exported .xml ready API file of project.

Use Project custom property and leave it empty while the project is saved into the repository or sharing with others.
One can pass the values at run time using testrunner using -P option.

You may also refer below post 

https://stackoverflow.com/questions/37655778/soapui-ability-to-switch-between-database-connections-for-test-suite/37659250#37659250