Gets rid of even some more null handling for FileLock

which is nowadays unnecessary. Also throws the expected IOException,
instead of an IllegalArgumentException if the lock couldn't be had from
the FileChannel. May bad, introduced in
0a5eb2aba58c9caf5061d88d20818157011a9add.
Merge pull request #1606 from tinwelint/1.9-expected-file-lock-exception

Gets rid of even some more null handling for FileLock