There was a Senate, but it's not as if it functioned as a proper governing body. Palpatine disbanded it a few years ago, but it's not as if he needed to do it officially.

[since after all, the Senate couldn't do much to reign in Palpatine's power, and largely served mostly to fool the majority of the galaxy that they had some input in the galactic government. The temperature cools slightly as Leia's focus shifts from Vader to the Senate, because while still personal, the Senate isn't as sensitive a subject. Not in the same way Vader is, at least. Her time as a senator feels like an entire lifetime ago, something she feels half detached from at this point.]