Fix sending invite emails

[elit0451]

Fixes: #11453

The issue occurs only on Umbraco Cloud due to a special "send email flow" which broke because of the change introduced in #11266. More specifically, there, the SMTP settings are not in the config files and sending null will make the emailMessage.From to always be null while the configuredEmailAddress won't also be setup:

Umbraco-CMS/src/Umbraco.Infrastructure/Extensions/EmailMessageExtensions.cs

Line 82 in f5ea5df

var fromEmail = string.IsNullOrEmpty(emailMessage.From) ? configuredFromAddress : emailMessage.From;

This PR makes sure to fall back to the current user's email if the SMTP settings are not configured.

[bergmania]

Changes looks fine and should be equivalent for non-cloud users :)