Is it obligatory to act upon things a person sees in a dream that later come true, provided they do not contradict Islamic law, such as an inspiration to perform a specific act of worship?

A righteous vision is from Allah Almighty, and it is among the glad tidings. A vision of someone commanding to pray Qiyam al-Layl (night prayer), and that its recipient will find much good in it, is a vision of goodness and blessing. Performing Qiyam al-Layl is legislated by the Qur'an, Sunnah, and scholarly consensus. It contains abundant goodness and great reward, and it is a discipline for the soul and health for the body. The vision of commanding Qiyam al-Layl is in the category of encouraging good, not obligating it.