[Improvement][UI] Set the default Tenant as default or the first exist tenant in save process

[chengshiwen]

*For better global communication, please give priority to using English description, thx! *

Please review https://dolphinscheduler.apache.org/en-us/docs/development/issue.html when describe an issue.

Describe the question
Steps to reproduce the behavior, for example:
0. Create a bare dolphinscheduler

1. Go to 'Create process'
2. Create a task
3. Save process
4. See default Tenant is 0
   

What are the current deficiencies and the benefits of improvement
Set the default Tenant as the first exist tenant

Which version of DolphinScheduler:
-[dev]

Describe alternatives you've considered
A clear and concise description of any alternative improvement solutions you've considered.