fix: comparing against a missing value

If a new package was added, the action would fail as it would try to
subtract `null` from an integer. Instead default to `0`.

compare.jq

@@ -2,7 +2,7 @@
 #
 # Where .[0] is the new value, and .[1] is the old value
 def calc_change($key):
-	. as $input | $input[0][$key] - $input[1][$key];
+	. as $input | ($input[0][$key] // 0) - ($input[1][$key] // 0);
 
 # Calculate the change in percentage for multiple keys
 #