Loading an empty file yields a nil (#906)

## Changes

Empty YAML files are valid and should return a nil-equivalent when
loaded.

## Tests

Tests pass.

libs/config/yamlloader/yaml.go

@@ -12,6 +12,9 @@ func LoadYAML(path string, r io.Reader) (config.Value, error) {
 	dec := yaml.NewDecoder(r)
 	err := dec.Decode(&node)
 	if err != nil {
+		if err == io.EOF {
+			return config.NilValue, nil
+		}
 		return config.NilValue, err
 	}
 

libs/config/yamlloader/yaml_test.go

@@ -28,3 +28,8 @@ func loadYAML(t *testing.T, path string) config.Value {
 	assert.EqualValues(t, ref, self.AsAny())
 	return self
 }
+
+func TestYAMLEmpty(t *testing.T) {
+	self := loadYAML(t, "testdata/empty.yml")
+	assert.Equal(t, config.NilValue, self)
+}