AppUtils#getBaseDirectory: be a bit more defensive
AppUtils#getBaseDirectory: be a bit more defensive

It is a mistake when the user passes null for a classLocation, but
we still can be nice even then.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

scijava/scijava-common