Fix support for empty units

When units are "", Unitful doesn't know what to and fails. It doesn't
fail normally, instead, it fails with a `MethodError`. This commit
treats that case specially.

Author: Sbozzolo

NEWS.md

@@ -39,6 +39,7 @@ julia> reordered_var.dims |> keys |> collect
   control the number of characters on each line of the legend of the box plot
 - Use default marker size instead of a marker size of 20 when plotting other models beside
   `CliMA` on the box plot
+- Fix support for `""` in units.
 
 v0.5.8
 ------

ext/ClimaAnalysisUnitfulExt.jl

@@ -15,6 +15,10 @@ Try converting `value` to a `Uniftul` object. If unsuccessful, just return it.
 """
 function Var._maybe_convert_to_unitful(value)
     value isa Unitful.Units && return value
+    # "" cannot be parsed but returns a wrong error (MethodError on lookup_units),
+    # so we handle it manually
+    value == "" && return value
+
     # This function in inherently type-unstable
     try
         return Unitful.uparse(value)

test/test_Var.jl

@@ -784,6 +784,13 @@ end
         data,
     )
 
+    var_empty_unit = ClimaAnalysis.OutputVar(
+        Dict{String, Any}("units" => ""),
+        Dict("long" => long),
+        dim_attributes,
+        data,
+    )
+
     @test ClimaAnalysis.has_units(var_with_unitful)
 
     # Convert to cm/s