Add omitempty on compose config top-level types

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

cli/command/stack/kubernetes/stackclient_test.go

@@ -32,10 +32,6 @@ services:
     image: bar
   foo:
     image: foo
-networks: {}
-volumes: {}
-secrets: {}
-configs: {}
 `), s.ComposeFile)
 }
 

cli/compose/types/types.go

@@ -73,10 +73,10 @@ type Config struct {
 	Filename string `yaml:"-"`
 	Version  string
 	Services Services
-	Networks map[string]NetworkConfig
-	Volumes  map[string]VolumeConfig
-	Secrets  map[string]SecretConfig
-	Configs  map[string]ConfigObjConfig
+	Networks map[string]NetworkConfig   `yaml:",omitempty"`
+	Volumes  map[string]VolumeConfig    `yaml:",omitempty"`
+	Secrets  map[string]SecretConfig    `yaml:",omitempty"`
+	Configs  map[string]ConfigObjConfig `yaml:",omitempty"`
 	Extras   map[string]interface{}     `yaml:",inline"`
 }