Set correct blink min fee (#701)

* min fee

* fix tests

* amount_split test

cashu/core/split.py

@@ -3,6 +3,8 @@ from typing import List
 
 def amount_split(amount: int) -> List[int]:
     """Given an amount returns a list of amounts returned e.g. 13 is [1, 4, 8]."""
+    if amount <= 0:
+        return []
     bits_amt = bin(amount)[::-1][:-2]
     rv = []
     for pos, bit in enumerate(bits_amt):