Allow icon() to work with empty namespace and/or prefix

[dereuromark]

Summary

• Makes namespace optional - null/empty values are now properly omitted instead of adding extra spaces
• Makes prefix optional - when null/empty, uses just the icon name without a prefix-dash

This allows using icon sets like Remixicon that don't use a namespace:

// Before: produced <i class="  ri-home-line"></i> (extra spaces)
// After: produces <i class="ri-home-line"></i>
$this->Html->icon('home-line', ['prefix' => 'ri', 'namespace' => null]);

Also supports hypothetical icon sets using just the icon name:

// Produces <icon class="home"></icon>
$this->Html->icon('home', ['tag' => 'icon', 'namespace' => null, 'prefix' => null]);

Fixes #429