`fusionblockstructure` with argument `TensorMapSpace` is not defined

Hi developers,

I find that when I tried to write
```
V = Z2Space(1=>1)
zeros(Float64,V←V)
```
as usual, it reports a bug saying
```
no method matching fusionblockstructure(::TensorMapSpace{GradedSpace{Z2Irrep, Tuple{Int64, Int64}}, 1, 1})
The function `fusionblockstructure` exists, but no method is defined for this combination of argument types.
```

