FIX: Don't try to generate Gargo.lock on empty workspaces.

FIX: Don't try to generate Gargo.lock on empty workspaces.

There was a panic! when the command `cargo update` was
executed in a workspace like this:

    mkdir ws
    cd ws
    echo '[workspace]' > Cargo.toml
    cargo new p1
    cargo new p2
    cargo update

The problem is that cargo tries to generate the Cargo.lock file
even if there aren't any members on the workspace.

This fix checks the existence of members in the workspace before trying
to do anything so at least we report an error instead of throwing a
panic!