I get a “null” value every time I take a photo of a license plate. For example, this one: https://upload.wikimedia.org/wikipedia/commons/4/4a/California_2018_License_Plate.jpg.
But honestly, that doesn’t even matter because the OCR API you’re using is not high quality enough to recognize license plates consistently.
I discuss that in a long thread linked from this post: Displaying the texts from the picture similar to the one from the textbox - #4 by tatiang.
The bottom line is that if you want to do license plate recognition, you’re going to have to pay for a better OCR API. I spent a lot of time trying to do what you’re doing with the same free API that you’re using so that I could help the users in those two topics. But I’m sorry to say that the free API you’re using is not worth your time.