How to set a default Profile in the User object?

Question

I'm setting up a process which will automatically create new Users, but need to set ProfileID to something. I can't use Process Builder because it's a required field and it would error out before it got to the Process, even if it's set to run on create.

It's an Informatica upsert function running off a list of employees. I can't hard code the profile ID into the Informatica task because some of the employess are already created and therefore already have assigned profiles.

The Profile I want to use is a custom profile. I don't think that matters, but I thought I'd mention it.

my current code (thank you @adrian) :

trigger AssignDefaultProfile on User (before insert) {

    static Profile fallbackProfile
        {
            get
                {
                    if (fallbackProfile == null)
                        fallbackProfile = [SELECT id FROM Profile p WHERE p.Name = 'Service Employee'] ; 
                    return fallbackProfile;
                }
            private set;
        }

    public static void setFallbackProfile(List<User> users)  
        {
            for (User user : users)
                if (user.ProfileId == null)
                    user.ProfileId = fallbackProfile.Id;
        }

    static UserRole fallbackRole
        {
            get
                {
                    if (fallbackRole == null)
                        fallbackRole = [SELECT id FROM UserRole r WHERE r.Name = 'General Employee'] ; 
                    return fallbackRole;
                }
            private set;
        }

    public static void setFallbackRole(List<User> users)
        {
            for (User user : users)
                if (user.UserRoleId == null)
                    user.UserRoleId = fallbackRole.Id;
        }
}

Answer 1

It would be simple enough to create a User trigger that sets the ProfileId value to your fallback if it is blank. Something along the lines of:

static Profile fallbackProfile
{
    get
    {
        if (fallbackProfile == null)
            fallbackProfile = [SELECT Id FROM Profile WHERE ...];
        return fallbackProfile;
    }
    private set;
}
public static void setFallbackProfile(List<User> users)
{
    for (User user : users)
        if (user.ProfileId == null)
            user.ProfileId = fallbackProfile.Id;
}

Just call the above in a before insert trigger on the User object and you should be good to go.

If you want to add a fallback UserRole as well, it would look something like:

static UserRole fallbackRole
{
    get
    {
        if (fallbackRole == null)
            fallbackRole = [SELECT Id FROM UserRole WHERE ...];
        return fallbackRole;
    }
    private set;
}
public static void setFallbackProfile(List<User> users)
{
    for (User user : users)
        if (user.UserRoleId == null)
            user.UserRoleId = fallbackRole.Id;
}

Best to keep the methods separate, since you don't want to populate the role just because the profile is blank, or vice versa.

Once you've added these methods to an Apex Class (I would call it UserService), you can reference them from your Apex Trigger.

trigger User on User (before insert)
{
    if (trigger.isBefore)
    {
        if (trigger.isInsert)
        {
            UserService.setFallbackProfile(trigger.new);
            UserService.setFallbackRole(trigger.new);
        }
    }
}

I highly recommend you look into trigger handler patterns and adopt one, the above is just to give you the idea.