[lldb] Fix that empty target.run-args are not actually used when launching process

`GetPropertyAtIndexAsArgs` returns true on success and false on failure. Right
now it returns the converted `size_t` returned from `GetArgs` which describes
the number of arguments in the argument list. So for empty argument lists
(`(size_t)0` -> `(bool)false`) this function always fails.

The only observable effect of this seems to be that empty arguments are never
propagated to the internal LaunchInfo for a process. This causes that once any
argument has been added to `target.run-args`, clearing `target.run-args` doesn't
have any effect.

Fixes issue #55568

Reviewed By: JDevlieghere, jingham

Differential Revision: https://reviews.llvm.org/D126057