Prevent adding empty constraints to the CQM model

Not sure if this is a bug or a feature request. Since I didn't expect this to happen, I file it as a bug, please change to feature request if more appropriate.

I do not expect to be able to add an empty constraint to a CQM model.
```
import dimod
cqm = dimod.CQM()
cqm.add_constraint( dimod.quicksum([]) <= 0)
print(len(cqm.constraints))
```
While there might be some rare use cases for this, like creating a constraint first and filling it later, it would be better to prevent this as the empty constraint might be counted toward maximum constraint allowed by the HSS system.
I also suggest to remove empty constraints resulted from variable fixing or constraint removing.

Start by reproducing the example through CQM.add_constraint with dimod.quicksum([]) <= 0, then inspect how empty constraints are represented. Check the variable-fixing and constraint-removing paths mentioned in the issue. Done means empty constraints are prevented or removed consistently and are not counted in cqm.constraints.

Written by the indexing model from the issue text.