Allow None for tunables using SolverStep
Allow None for tunables using SolverStep

When y returns None, a SolverStep object skips this tunable for this
call. This allows us to skip if the tunable's y is currently invalid or
inaccessible which can be the case often in the start of a run or some
other condition.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue