Don't set platform defaults on OSConfig struct immediately

Instead, query the platform defaults only if the config is empty. This will be
necessary later to distinguish an empty config from a default config, so that we
can give deprecation warnings.

pkg/commands/oscommands/os.go

@@ -79,6 +79,9 @@ func FileType(path string) string {
 
 func (c *OSCommand) OpenFile(filename string) error {
 	commandTemplate := c.UserConfig.OS.OpenCommand
+	if commandTemplate == "" {
+		commandTemplate = config.GetPlatformDefaultConfig().OpenCommand
+	}
 	templateValues := map[string]string{
 		"filename": c.Quote(filename),
 	}
@@ -88,6 +91,9 @@ func (c *OSCommand) OpenFile(filename string) error {
 
 func (c *OSCommand) OpenLink(link string) error {
 	commandTemplate := c.UserConfig.OS.OpenLinkCommand
+	if commandTemplate == "" {
+		commandTemplate = config.GetPlatformDefaultConfig().OpenLinkCommand
+	}
 	templateValues := map[string]string{
 		"link": c.Quote(link),
 	}

pkg/config/user_config.go

@@ -599,7 +599,7 @@ func GetDefaultConfig() *UserConfig {
 				BulkMenu: "b",
 			},
 		},
-		OS:                           GetPlatformDefaultConfig(),
+		OS:                           OSConfig{},
 		DisableStartupPopups:         false,
 		CustomCommands:               []CustomCommand(nil),
 		Services:                     map[string]string(nil),