Fix initial era in staking

[gui1117]

Fix #8911

Breaking change

The election for the initial era must be successful to create the initial era.
In the past a failing initial election would generate an initial era with no stakers
Now a failing initial election will generate no initial era.

Done

• refactor test in order not to use this initial era created from failing election.

To do

• look at pallet session if no initial era can result in unexpected logic.

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.