The rule 503.1 states that there are no turn-based actions in the Upkeep step. However, 703.4p applies at the end of each step and phase. These two statements seem contradictory, could I get a clarification, please?

503.1. The upkeep step has no turn-based actions. Once it begins, the active player gets priority. (See rule 116, “Timing and Priority.”)

703.4p. When each step or phase ends, any unused mana left in a player's mana pool empties. See rule 500.4.