When calling GC#setFont(), a copy of the given font is created. However,
the font may be null, leading to an exception. In addition, a font
should be retrieved from the font provider instead of creating a new
font that is manually disposed later on.

This change ensures that null fonts are properly handled and that the
stored font is retrieved from the provider.

Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/2350