[LLDB] Fix the use of "platform process launch" with no extra arguments

This fixes #62068.

After 8d1de7b3 the following issue appeared:
```
$ ./bin/lldb /tmp/test.o
(lldb) target create "/tmp/test.o"
Current executable set to '/tmp/test.o' (aarch64).
(lldb) platform process launch -s
error: Cannot launch '': Nothing to launch
```

Previously would call target->GetRunArguments when there were no extra
arguments, so we could find out what target.run-args might be.

Once that change started relying on the first arg being the exe,
the fact that that call clears the existing argument list caused the bug.

Instead, have it set a local arg list and append that to the existing
one. Which in this case will just contain the exe name.

Since there's no existing tests for this command I've added a new file
that covers enough to check this issue.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D153636