Do not set _is_defined to true if there are no DoFs.

Closes #4330
Merge pull request #4331 from andrsd/is-defined-fix

Do not set _is_defined to true if there are no DoFs.
Fixing computeNodalValues()

When the _is_defined flag was turned from on to off, the _nodal_u et
al. variables remained resized to what they were before. That caused
the code to use those garbagge values.

Refs #4330
Introducing a side-effect that Richard's module relied on

When Richard's module is migrated to the NodalCoupleable interface,
reverting this patch should remove the side-effect and all will be good
again.

Refs #4038, #4330
Merge pull request #4340 from andrsd/parallel-fix

Fixing MooseVariable::computeNodalValues
Merge commit 'a1b19298c0ba670e11a18b6f600a617a00e0d077'