target: ensure that each used target has NewXXXTargetResult defined

Ensure that `UnmarshalTargetResultOptions()` is called only when there
are any options to unmarshal in the JSON object.

Since results of some Targets don't have any options defined, mark
`TargetResult.Options` as optional in the JSON tag.

internal/target/azure_target.go

@@ -24,3 +24,7 @@ func (AzureTargetOptions) isTargetOptions() {}
 func NewAzureTarget(options *AzureTargetOptions) *Target {
 	return newTarget(TargetNameAzure, options)
 }
+
+func NewAzureTargetResult() *TargetResult {
+	return newTargetResult(TargetNameAzure, nil)
+}