fix: return proper value from Bridge.STP instead of plain nil

Since we are returning interfaces in config, we have to return something typed for method chaining to work. Otherwise,
it simply doesn't know what method to call because there is no type information. We also don't want to change
the default config behavior, so we don't try to check for `nil` after calling `.STP()`.

Closes #8626

Signed-off-by: Dmitriy Matrenichev <dmitry.matrenichev@siderolabs.com>

pkg/machinery/config/types/v1alpha1/v1alpha1_provider.go

@@ -1047,7 +1047,7 @@ func (b *Bridge) Interfaces() []string {
 // STP implements the config.Bridge interface.
 func (b *Bridge) STP() config.STP {
 	if b.BridgeSTP == nil {
-		return nil
+		return (*STP)(nil)
 	}
 
 	return b.BridgeSTP