Don't overwrite password fields on connection edit

Problem/Motivation

When you edit a connection plugin with a password, it will show an empty password field and because the submitted config is saved as-is, the stored password is cleared/overwritten with an empty string on save.

Steps to reproduce

Use the Mail Group IMAP plugin, setup an IMAP server connection, edit and save it again.

Proposed resolution

Post form validation:

• Check for password fields, that are not flagged as required
• Remove corresponding fields with empty value in the form state, from the form state
• Merge the form state values with the current config before save

This assumes the connection plugin flags password fields as not required if a value already exists. If not, then it forces the end-user to provide the password(s) anyway and nothing should change.