Tests: unnecessary checks for null

[jottinger]

I added jacoco to a local fork to see about test coverage, and there are paths untested entirely (FileHandler.readFileMap() for example) or other paths that should not be accessible (PathHandler.getDirectoryPath() has a path where object is null, for example, and that can be replaced with Objects.requireNonNull(object;) if you're on a recent JDK.

[jottinger]

This should be dependent on issue #5 being merged, as some of the tests want slightly different APIs; in particular, @Test void testHere() throws IOException { ... } wants to be replaced with @Test void testHere() { try { doStuffThatThrowsException(); } catch(IOException e) { fail(e); } } instead.