feat: allow users not to provide config option with default values

Now, you are able to run the tracler like this:

```
TORRUST_TRACKER_CONFIG="" cargo run
```

Default values will be used for the missing values in the provided
configuration. In that case, none of the values have been provided, so
it will use default values for all options.

packages/configuration/src/v1/mod.rs

@@ -239,7 +239,7 @@ use std::fs;
 use std::net::IpAddr;
 use std::str::FromStr;
 
-use figment::providers::{Env, Format, Toml};
+use figment::providers::{Env, Format, Serialized, Toml};
 use figment::Figment;
 use serde::{Deserialize, Serialize};
 use torrust_tracker_primitives::{DatabaseDriver, TrackerMode};
@@ -393,7 +393,7 @@ impl Configuration {
     ///
     /// Will return `Err` if the environment variable does not exist or has a bad configuration.
     pub fn load(info: &Info) -> Result<Configuration, Error> {
-        let figment = Figment::new()
+        let figment = Figment::from(Serialized::defaults(Configuration::default()))
             .merge(Toml::string(&info.tracker_toml))
             .merge(Env::prefixed("TORRUST_TRACKER__").split("__"));
 
@@ -523,6 +523,24 @@ mod tests {
         assert_eq!(contents, default_config_toml());
     }
 
+    #[test]
+    fn configuration_should_use_the_default_values_when_an_empty_configuration_is_provided_by_the_user() {
+        figment::Jail::expect_with(|_jail| {
+            let empty_configuration = String::new();
+
+            let info = Info {
+                tracker_toml: empty_configuration,
+                api_admin_token: None,
+            };
+
+            let configuration = Configuration::load(&info).expect("Could not load configuration from file");
+
+            assert_eq!(configuration, Configuration::default());
+
+            Ok(())
+        });
+    }
+
     #[test]
     fn configuration_should_be_loaded_from_a_toml_config_file() {
         figment::Jail::expect_with(|_jail| {