fix format_satoshis for None values

lib/util.py

@@ -109,6 +109,8 @@ def user_dir():
 
 def format_satoshis(x, is_diff=False, num_zeros = 0, decimal_point = 8, whitespaces=False):
     from locale import localeconv
+    if x is None:
+        return 'unknown'
     x = int(x)  # Some callers pass Decimal
     scale_factor = pow (10, decimal_point)
     integer_part = "{:n}".format(int(abs(x) / float(scale_factor)))