rust: pl011: Allow NULL chardev argument to pl011_create()

rust: pl011: Allow NULL chardev argument to pl011_create()

It's valid for the caller to pass a NULL chardev to pl011_create();
this means "don't set the chardev property on the device", which
in turn means "act like there's no chardev". All the chardev
frontend APIs (in C, at least) accept a NULL pointer to mean
"do nothing".

This fixes some failures in 'make check-functional' when Rust support
is enabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250307190051.3274226-1-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>