Must fully specify RegistrySpec

If you pass a RegistrySpec without a name (i.e., just set the url) then CompatHelper runs into this error:
```
ERROR: ArgumentError: `nothing` can not be interpolated into commands (`Cmd`)
Stacktrace:
[1] cmd_interpolate1(::Nothing) at ./cmd.jl:321
[2] map(::typeof(Base.cmd_interpolate1), ::Tuple{Nothing}) at ./tuple.jl:157
[3] cmd_interpolate(::Nothing) at ./cmd.jl:319
[4] arg_gen(::Nothing) at ./cmd.jl:340
[5] cmd_gen(::Tuple{Tuple{SubString{String}},Tuple{SubString{String}},Tuple{String},Tuple{Nothing}}) at ./cmd.jl:366
[6] get_latest_version_from_registries!(::Dict{CompatHelper.Package,Union{Nothing, VersionNumber}}, ::Array{Pkg.Types.RegistrySpec,1}) at /home/runner/.julia/packages/CompatHelper/COjxl/src/get_latest_version_from_registries.jl:17
[7] main(::Function, ::Base.EnvDict, ::CompatHelper.GitHubActions; registries::Array{Pkg.Types.RegistrySpec,1}, keep_existing_compat::Bool, drop_existing_compat::Bool, master_branch::CompatHelper.DefaultBranch, pr_title_prefix::String, subdirs::Array{String,1}) at /home/runner/.julia/packages/CompatHelper/COjxl/src/main.jl:45
[8] top-level scope at none:1
```

The workaround is fully specifying the RegistrySpec. It seems to me like I should be able to just specify the RegistrySpec url. The name and uuid should be able to be inferred from that.