rose mpi-launch: fix unbound variable

UM reconfiguration jobs in my first UM Rose suite (now working!) were failing with:

```
/oper/admin/um_fcm/rose/rose.git/bin/rose-mpi-launch: line 229: ROSE_LAUNCHER_BASE: unbound variable
[FAIL] um-recon # return-code=1
```

I'm not entirely sure if this is a bug or if just reflects some deficiency in my local rose installation, but initializing the variable if unset fixed it for me...