Fix issues introduced in 8bcdff4

[MarcMichalsky]

In commit 8bcdff4, some of the logic for testing whether or not $profile->getAttribute() returns a configuration value was changed to use isset() or is_string() instead of !empty().

The problem is that $profile->getAttribute() looks for the attribute name in it's $data property, which also contains the attribute names for config values that are not set (the value is then an empty string).

I suggest changeing $profile->getAttribute() as follows:

public function getAttribute($attribute_name, $default = NULL) {
    return $this->data[$attribute_name] ?? $default;
}

public function getAttribute($attribute_name, $default = NULL) {
    return !empty($this->data[$attribute_name]) ? $this->data[$attribute_name] : $default;
}

Or should we just revert commit 8bcdff4 instead?

[jensschuppe]

Hm, seems my assumptions weren't correct everywhere. The point of those changes was to replace the use of empty(), so introducing empty() again does not seem right. We don't want to use empty() in general, as it's too broad of a comparison and does not take types into account.

Could you list the insufficient comparisons you noticed in 8bcdff4? I'll have a look at them again …