fix Transition for empty model

Author: penelopeysm

src/mcmc/Inference.jl

@@ -166,15 +166,23 @@ struct Transition{T,F<:AbstractFloat,N<:NamedTuple} <: AbstractTransition
         logprior = DynamicPPL.getlogprior(vi)
         loglikelihood = DynamicPPL.getloglikelihood(vi)
 
-        # Convert values to the format needed (with individual VarNames split up).
-        # TODO(penelopeysm): This wouldn't be necessary if not for MCMCChains's poor
-        # representation...
-        iters = map(
-            DynamicPPL.varname_and_value_leaves,
-            keys(vals_as_in_model),
-            values(vals_as_in_model),
-        )
-        values_split = mapreduce(collect, vcat, iters)
+        # Convert values to the format needed (i.e. a Vector of (varname,
+        # value) tuples, where value isa Real: all vector-valued varnames must
+        # be split up.)
+        # TODO(penelopeysm): This wouldn't be necessary if not for MCMCChains's
+        # poor representation...
+        values_split = if isempty(vals_as_in_model)
+            # If there are no values, we return an empty vector.
+            # This is the case for models with no parameters.
+            Vector{Tuple{VarName,Any}}()
+        else
+            iters = map(
+                DynamicPPL.varname_and_value_leaves,
+                keys(vals_as_in_model),
+                values(vals_as_in_model),
+            )
+            mapreduce(collect, vcat, iters)
+        end
 
         # Get additional statistics
         stats = getstats(sampler_transition)