An empty mailcard is not switched by changing identity

By creating a new mail from a shared mailbox using your identity with an empty mailcard, if you want to change your identity with a personal one, the associated signature is not shown.

Steps to reproduce:

• user1 (with empty mailcard) shares his account with user2 also sharing identity and mailcard
• user2 has its own mailcard configured
• login with user2 select a folder of the account user1
• new email with user1 identity (without mailcard)
• switch identity and select the personal one of user2

Expected behaviour:

• user2 mailcard is shown

Actual behaviour:

• user2 mailcard is not shown

The problem appeared after the upgrade to the 5.10.0 WT-release

Thanks to Angelo Bevilacqua (Ref. #126314)