Fix concatentation error on no args to `./mach rustc`

Since default argument to params is None, concatenating it with a
list will raise an error.  This behaviour prevents `./mach rustc`
to be called when system-rust is defined in .servobuild.

Currently it will only work when followed by an argument, i.e.
`./mach rustc -arg`.

Testing this patch: `./mach rustc` should not raise an error.