ScopedValues: print valid code for undef value (#59478)
Seemed a bit odd to me for the printing of ScopedValues without a
default to not print as valid code, when that seems perfectly clear.
Before (on `master`):
```julia
julia> ScopedValue{Int}()
ScopedValue{Int64}(undefined)
julia> ScopedValue{String}()
ScopedValue{String}(undefined)
```
Now (this PR):
```julia
julia> ScopedValue{Int}()
ScopedValue{Int64}()
julia> ScopedValue{String}()
ScopedValue{String}()
```
