What happens if my rune deck is empty and I cannot channel anymore runes?

If your rune deck is empty and you are instructed to channel runes, you simply do not channel any runes.

According to RiftJudge FAQ #5452: "No, you do not draw a card if you cannot channel. You simply do not channel at all."

Additionally, Rule 315.3.b.1 states: "If there are fewer than 2 runes in the Rune Deck, they channel as many as possible." If there are zero runes in your deck, you cannot channel any, and the game continues without penalty or additional card draws, unless a specific card effect explicitly states otherwise (as noted in RiftJudge FAQ #3355).