Maintain consistent JSON formatting

The JSON gem has historically included newlines when pretty printing
empty arrays or hashes. This changed with ruby/json@b2c4480 in JSON
2.8.0.

In order to maintain consistent behavior for our users, this commit
special cases empty array and hash facts and adds a new test for empty
hashes.

lib/puppet/face/facts.rb

@@ -167,7 +167,15 @@
 
       case result
       when Array, Hash
-        Puppet::Util::Json.dump(result, :pretty => true)
+        # JSON < 2.8.0 would pretty print empty arrays and hashes with newlines
+        # Maintain that behavior for our users for now
+        if result.is_a?(Array) && result.empty?
+          "[\n\n]"
+        elsif result.is_a?(Hash) && result.empty?
+          "{\n}"
+        else
+          Puppet::Util::Json.dump(result, :pretty => true)
+        end
       else # one of VALID_TYPES above
         result
       end

spec/unit/application/facts_spec.rb

@@ -91,6 +91,7 @@
 
     {
       "type_hash" => [{'a' => 2}, "{\n  \"a\": 2\n}"],
+      "type_empty_hash" => [{}, "{\n}"],
       "type_array" => [[], "[\n\n]"],
       "type_string" => ["str", "str"],
       "type_int" => [1, "1"],