Fix a bug in lldb-dotest that was uncovered by setting no value for dotest_args_str.

We were splitting the string, and adding that array to cmd.  But split generated
[''] which shows up later on as an empty "test directory search path".  That got
extended to the CWD + "" which generally doesn't have any tests, so

lldb-dotest -p SomeRealTest.py

would fail with a no matching tests error.

Differential Revision: https://reviews.llvm.org/D133075