Prettify recent change

Rather than pretend that passing a null to getBaseDirectory(File,
String) does anything (it does not: it checks the first parameter
whether it is null, first thing, and returns null if that is the
case), we can write the code in a more concise manner that at least
this developer has an easier time to understand.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>