Allow leaving out initial_stdin setting in config file

nix/module.nix

@@ -131,10 +131,10 @@ in
             '';
           };
           initial_stdin = lib.mkOption {
-            type = lib.types.str;
-            default = "";
+            type = lib.types.nullOr lib.types.str;
+            default = null;
             example = lib.literalExpression ''
-              if cfg.localhost then "server ::1\n" else "";
+              if cfg.localhost then "server ::1\n" else null;
             '';
             description = lib.mdDoc ''
               String to send to the stdin of the update program before sending anything else.
@@ -224,7 +224,8 @@ in
       startLimitBurst = 1;
 
       serviceConfig = let
-        settingsFile = settingsFormat.generate "dyndnsd.toml" cfg.settings;
+        settingsNoNulls = lib.filterAttrsRecursive (_: v: v != null) cfg.settings;
+        settingsFile = settingsFormat.generate "dyndnsd.toml" settingsNoNulls;
         runtimeConfigPath = if cfg.environmentFiles != []
           then "/run/${RuntimeDirectory}/dyndnsd.toml"
           else settingsFile;

src/config.rs

@@ -39,7 +39,7 @@ pub struct Config<'a> {
 pub struct UpdateProgram {
 	pub bin: String,
 	pub args: Vec<String>,
-	pub initial_stdin: String,
+	pub initial_stdin: Option<String>,
 	pub stdin_per_zone_update: String,
 	pub final_stdin: String,
 	pub ipv4: SpecialUpdateProgram,

src/process.rs

@@ -34,7 +34,9 @@ fn splice_ipv6_addrs(prefixlen: u8, prefix: Ipv6Addr, suffix: Ipv6Addr) -> Ipv6A
 fn build_command_string(config: &Config, user: &User, q: &QueryParameters) -> String {
 	// TODO: stream stdin to the process instead of building a string and then pushing it all at once
 	let mut command = String::new();
-	command.push_str(config.update_program.initial_stdin.as_str());
+	if let Some(initial_stdin) = &config.update_program.initial_stdin {
+		command.push_str(initial_stdin);
+	}
 	let domains = &user.domains;
 	for (domain, props) in domains {
 		trace!("Domain: {domain:?} {props:?}");