BaseApp get.PreferenceGroup fails if PreferenceGroup is not set

[rjackey]

Due to PreferenceGroup being a string type, need to change get.PreferenceGroup away from empty char format:

    function value = get.PreferenceGroup(app)
        value = app.PreferenceGroup;
        if isempty(value)
            value = class(app);
        end
        value = matlab.lang.makeValidName(value);
    end

to:

    function value = get.PreferenceGroup(app)
        value = app.PreferenceGroup;
        if ~strlength(value)
            value = class(app);
        end
        value = matlab.lang.makeValidName(value);
    end