Use compact, not flatten, in the model registry (#6737)

`#safe_constantize` shouldn’t ever return an Array, so there shouldn’t be
any need for flattening. It may, however, return `nil`. `#compact` will
get rid of that, and likely is what was intended here.

app/models/hyrax/model_registry.rb

@@ -99,7 +99,7 @@ def self.work_rdf_representations
     end
 
     def self.classes_from(strings)
-      strings.map(&:safe_constantize).flatten.uniq
+      strings.map(&:safe_constantize).compact.uniq
     end
     private_class_method :classes_from
 

spec/models/hyrax/model_registry_spec.rb

@@ -29,4 +29,11 @@
       it { is_expected.to all(be_kind_of(String)) }
     end
   end
+
+  describe ".classes_from" do
+    subject { described_class.send("classes_from", ["DefinitelyNotARealClass", "Hyrax"]) }
+
+    it { is_expected.to be_a(Array) }
+    it { is_expected.not_to include(nil) }
+  end
 end