fix bug with loading plugins in shim with no config (#7816)

plugins/common/shim/config.go

@@ -146,7 +146,11 @@ func createPluginsWithTomlConfig(md toml.MetaData, conf config) (loadedConfig, e
 // have registered themselves with the registry. This makes loading plugins
 // without having to define a config dead easy.
 func DefaultImportedPlugins() (config, error) {
-	conf := config{}
+	conf := config{
+		Inputs:     map[string][]toml.Primitive{},
+		Processors: map[string][]toml.Primitive{},
+		Outputs:    map[string][]toml.Primitive{},
+	}
 	for name := range inputs.Inputs {
 		conf.Inputs[name] = []toml.Primitive{}
 		return conf, nil

plugins/common/shim/config_test.go

@@ -27,3 +27,13 @@ func TestLoadConfig(t *testing.T) {
 	require.Equal(t, "xxxxxxxxxx", inp.SecretToken)
 	require.Equal(t, `test"\test`, inp.SecretValue)
 }
+
+func TestDefaultImportedPluginsSelfRegisters(t *testing.T) {
+	inputs.Add("test", func() telegraf.Input {
+		return &testInput{}
+	})
+
+	cfg, err := LoadConfig(nil)
+	require.NoError(t, err)
+	require.Equal(t, "test", cfg.Input.Description())
+}

plugins/common/shim/input_test.go

@@ -85,7 +85,7 @@ func (i *testInput) SampleConfig() string {
 }
 
 func (i *testInput) Description() string {
-	return ""
+	return "test"
 }
 
 func (i *testInput) Gather(acc telegraf.Accumulator) error {