plugins: Ensure register handles are not NULL

plugins: Ensure register handles are not NULL

Ensure register handles are not NULL so that a plugin can assume NULL is
invalid as a register handle.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240229-null-v1-1-e716501d981e@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240620152220.2192768-4-alex.bennee@linaro.org>