Improve handling of empty valid yaml files (#716)

Improves the way empty valid `yaml` files are handled.
When deserializing a `config` or `layout` file, that is
an empty valid `yaml` file, eg:

```
---
```

We now assume the default configuration is desired.

zellij-utils/src/input/config.rs

@@ -99,17 +99,22 @@ impl TryFrom<&CliArgs> for Config {
 impl Config {
     /// Uses defaults, but lets config override them.
     pub fn from_yaml(yaml_config: &str) -> ConfigResult {
-        let config_from_yaml: ConfigFromYaml = serde_yaml::from_str(yaml_config)?;
+        let config_from_yaml: Option<ConfigFromYaml> = serde_yaml::from_str(yaml_config)?;
-        let keybinds = Keybinds::get_default_keybinds_with_config(config_from_yaml.keybinds);
-        let options = Options::from_yaml(config_from_yaml.options);
-        let themes = config_from_yaml.themes;
 
+        match config_from_yaml {
+            None => Ok(Config::default()),
+            Some(config) => {
+                let keybinds = Keybinds::get_default_keybinds_with_config(config.keybinds);
+                let options = Options::from_yaml(config.options);
+                let themes = config.themes;
                 Ok(Config {
                     keybinds,
                     options,
                     themes,
                 })
             }
+        }
+    }
 
     /// Deserializes from given path.
     pub fn new(path: &Path) -> ConfigResult {

zellij-utils/src/input/layout.rs

@@ -103,9 +103,12 @@ impl LayoutFromYaml {
 
         let mut layout = String::new();
         layout_file.read_to_string(&mut layout)?;
-        let layout: LayoutFromYaml = serde_yaml::from_str(&layout)?;
+        let layout: Option<LayoutFromYaml> = serde_yaml::from_str(&layout)?;
 
-        Ok(layout)
+        match layout {
+            Some(layout) => Ok(layout),
+            None => Ok(LayoutFromYaml::default()),
+        }
     }
 
     // It wants to use Path here, but that doesn't compile.