Fix type comparison printing in `MethodError`s from convert. (#49645)
This patch removes the erroneous `{}` for types without parameters when
printing `MethodError`s from convert. Fixes e.g. the following (note
`Float64{}`):
```julia
julia> struct A{B, C} end
julia> convert(A{Float64,1}, A{Float64,2}())
ERROR: MethodError: Cannot `convert` an object of type
A{Float64{},2} to an object of type
A{Float64{},1}
```