target directory "null" when using build-helper-maven-plugin to add test sources

[Benson Margulies]

I have no test sources in the main test directory, but I use the build help
to add some. And I get ...

[INFO] [compiler:testCompile]
[INFO] Compiling 6 source files to
/Users/benson/x/trunk/rlp/lucene/l23s13/null/test-classes