Call action widget should accept empty input

[shuse2]

Expected behavior

Call action widget should be able to be used without input in case where input can be empty.
Also, it should not throw error on console silently

Actual behavior

When input can be empty, such as getLastBlock, when pressing Submit, it throws error on console.
Also, there should be placeholder with {}

Steps to reproduce

Call getLastBlock in dashboard plugin

Which version(s) does this affect? (Environment, OS, etc...)

feature/6216-add_dashboard-plugin