Omit empty bundle fields

We were always writing out some bundle fields, like both path and the
environment variable when it's always an either/or situation with those
fields.

pkg/config/manifest.go

@@ -52,13 +52,13 @@ type ParameterDefinition struct {
 
 type CredentialDefinition struct {
 	Name                string `yaml:"name"`
-	Path                string `yaml:"path"`
-	EnvironmentVariable string `yaml:"env"`
+	Path                string `yaml:"path,omitempty"`
+	EnvironmentVariable string `yaml:"env,omitempty"`
 }
 
 type Location struct {
-	Path                string `yaml:"path"`
-	EnvironmentVariable string `yaml:"env"`
+	Path                string `yaml:"path,omitempty"`
+	EnvironmentVariable string `yaml:"env,omitempty"`
 }
 
 // ParameterMetadata contains metadata for a parameter definition.

pkg/porter/bundle.go

@@ -74,18 +74,18 @@ type InvocationImage struct {
 //
 // A location may be either a file (by path) or an environment variable.
 type Location struct {
-	Path                string `json:"path" toml:"path"`
-	EnvironmentVariable string `json:"env" toml:"env"`
+	Path                string `json:"path,omitempty" toml:"path"`
+	EnvironmentVariable string `json:"env,omitempty" toml:"env"`
 }
 
 // Maintainer describes a code maintainer of a bundle
 type Maintainer struct {
 	// Name is a user name or organization name
 	Name string `json:"name" toml:"name"`
 	// Email is an optional email address to contact the named maintainer
-	Email string `json:"email" toml:"email"`
+	Email string `json:"email,omitempty" toml:"email"`
 	// Url is an optional URL to an address for the named maintainer
-	URL string `json:"url" toml:"url"`
+	URL string `json:"url,omitempty" toml:"url"`
 }
 
 // Action describes a custom (non-core) action.