Default to name if realName is not available

Currently, the ComboBox that shows the selected user has textRole set to "realName", meaning it shows the real name of a user (from the gecos field in /etc/passwd, which can be set with chfn --full-name). However, it's not true that this field is necessarily set.

A user who does not set the real name attribute will have an empty name, displaying nothing. Instead, we should default to showing the username if realName is empty.

Normally I would use the getValue() function in SimpleControls/ComboBox.qml to get the username. However, the first time getValue() is called (i.e. before any user is selected), it throws an index error. Running sddm-greeter --test-mode --theme /usr/share/sddm/themes/simplicity:

Main.qml:152:5: QML ComboBox (parent or ancestor of VisualDataGroup): get: index out of range
Main.qml:48:9: QML ComboBox (parent or ancestor of VisualDataGroup): get: index out of range

This index error is particularly strange because control.delegateModel.items.count is right, so it should be accessible.

Because of this index error, I had to wrap getValue() in a try-catch, and return userModel.lastUser on an error, which is the string username of the last user selected. This is a reasonable default because as stated previously, the error only occurs when a user has not been selected, in which case the theme defaults to selecting the last user.

In conclusion,

• on initial startup, if the last user has a non-empty real name, displayText is used directly
• otherwise, getValue() is called, which throws an error, so userModel.lastUser is used (which should match the last user)
• if the user then selects an account with a non-empty real name, displayText is used directly
• otherwise, getValue() is called, which properly returns the username