runner: Concatenating null strings in Java (JDK 1.5+)

JLS, Section 15.18.1.1:
If the reference is null, it is converted to the string "null".
http://docs.oracle.com/javase/specs/jls/se5.0/html/expressions.html#15.18.1.1

Change-Id: Idae09c02b24da38ec6f3475fd0af35ef48a2405b
Reviewed-on: https://gerrit.libreoffice.org/11930
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>