User shouldn't be allowed to save settings with "JIRA home" entry empty in config tool.

Steps:
1. JIRA 4.0 onwards JIRA_Home is a mandatory entry for JIRA to work properly.
2. Launch config.sh and delete the existing JIRA_Home value. Click "Save".

Result:
1. It save the setting without any issues.

Expected:
1. It should give error message as JIRA_home entry is required for JIRA to work.