Bounds Error when no cut cells are present

@fverdugo @santiagobadia
If we consider the following code
```
using Gridap
using GridapEmbedded
using Gridap.ReferenceFEs

u(x) = x[1] + x[2]
f(x) = 0.0

bgdomain = (0,1,0,1)
n = 10
partition = (n,n)
bgmodel = CartesianDiscreteModel(bgdomain,partition)

geo = square(L=0.8,x0=Point(0.5,0.5))
cutgeo = cut(bgmodel,geo)
model = DiscreteModel(cutgeo)

strategy = AggregateCutCellsByThreshold(1.0)
aggregates = aggregate(strategy,cutgeo)

T = Float64
order = 1
reffe = LagrangianRefFE(T,QUAD,order)
V0 = TestFESpace(reffe=reffe, model=model,dof_space=:physical)
V = AgFEMSpace(V0,aggregates)
U = TrialFESpace(V)
```
Here, all cells are well-posed (no cut cells).

We can see an error **BoundsError: attempt to access 81-element Array{Int32,1} at index [0]** in the line
`V = AgFEMSpace(V0,aggregates)`.