Is seeing the Prophet (peace be upon him) in a dream in the form of a shadow considered not seeing him, or is it a valid vision as long as the dreamer knows it is him?

The Prophet has physical characteristics mentioned in the books of Shama'il (characteristics). So, whoever sees him with these characteristics has truly seen him, because Satan cannot imitate them. Since what was seen is a shadow, it is not possible to definitively confirm whether it was the Prophet's shadow or not, but it is hoped that it was.