Issue 529: Create null Latent model (#530)

* Null Latent model

* Null Latent model

* fix doctest

EpiAware/src/EpiLatentModels/EpiLatentModels.jl

@@ -15,7 +15,7 @@ using Turing, Distributions, DocStringExtensions, LinearAlgebra, SparseArrays,
       OrdinaryDiffEq
 
 #Export models
-export FixedIntercept, Intercept, RandomWalk, AR, HierarchicalNormal
+export FixedIntercept, Intercept, RandomWalk, AR, HierarchicalNormal, Null
 
 #Export ODE definitions
 export SIRParams, SEIRParams
@@ -38,6 +38,7 @@ include("models/Intercept.jl")
 include("models/RandomWalk.jl")
 include("models/AR.jl")
 include("models/HierarchicalNormal.jl")
+include("models/Null.jl")
 include("odemodels/SIRParams.jl")
 include("odemodels/SEIRParams.jl")
 include("modifiers/DiffLatentModel.jl")

EpiAware/src/EpiLatentModels/models/Null.jl

@@ -0,0 +1,24 @@
+@doc raw"
+A null model struct. This struct is used to indicate that no latent variables are to be generated.
+"
+struct Null <: AbstractTuringLatentModel end
+
+@doc raw"
+Generates `nothing` as latent variables for the given `latent_model` of type `Null`.
+
+# Example
+```jldoctest
+using EpiAware
+null = Null()
+null_mdl = generate_latent(null, 10)
+isnothing(null_mdl())
+
+# output
+
+true
+
+```
+"
+@model function EpiAwareBase.generate_latent(latent_model::Null, n)
+    return nothing
+end

EpiAware/test/EpiLatentModels/models/Null.jl

@@ -0,0 +1,8 @@
+@testitem "Null Model Tests" begin
+    # Test that Null can be instantiated
+    @test Null() isa Null
+
+    # Test that generate_latent returns nothing
+    null = Null()
+    @test isnothing(generate_latent(null, 10)())
+end