Stop appendReplacement from throwing an exception when a replacement group is null.

[jkadams]

See bug 24605755.

[jkadams]

In that case, start and end both return -1:
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Matcher.html#start(int)
"or -1 if the match was successful but the group itself did not match anything"

Tested with java.util.regex.Matcher, and start/end both return -1 and group returns null, so this is fine the way it is - the only bug is in appendReplacement.

[alandonovan]

Thanks for checking.