NullPointerExceptions in vote parser

While looking a bit at the logs I see recurring NullPointerExceptions we should fix:

2024-08-23 06:07:12,987 WARN o.t.m.d.Main:171 VoteParser run().
java.lang.NullPointerException: Cannot invoke "String.length()" because "s" is null
	at java.base/java.util.Formatter.parse(Formatter.java:2717)
	at java.base/java.util.Formatter.format(Formatter.java:2671)
	at java.base/java.util.Formatter.format(Formatter.java:2625)
	at java.base/java.lang.String.format(String.java:4145)
	at org.torproject.metrics.descriptorparser.parsers.VoteParser.addNetworkStatusVote(VoteParser.java:250)
	at org.torproject.metrics.descriptorparser.parsers.VoteParser.run(VoteParser.java:92)
	at org.torproject.metrics.descriptorparser.Main.run(Main.java:169)
	at org.torproject.metrics.descriptorparser.Main.exec(Main.java:37)
	at org.torproject.metrics.descriptorparser.Main.main(Main.java:33