Answer in Details
If the rice fell accidentally while you were preparing the offerings, it is generally not considered inauspicious in Lord Shiva’s philosophy. Shiva is known as "Bholenath," the innocent and easily pleased Lord. He values devotion and intention more than ritualistic perfection. Mistakes made unintentionally during worship are forgiven, as long as your heart is pure and your devotion sincere.
If you feel concerned, you may respectfully collect the fallen rice and place it at the feet of the Lingam or outside under a tree as an offering to nature. You can also pray to Lord Shiva, expressing your intentions and seeking forgiveness for any unintentional mistake. Remember, Shiva looks at the purity of your heart, not the perfection of your actions.
Om Namah Shivaya.