System properties should be optional

[boozook]

[There][] are optional properties and empty by default in almost all templates what I have seen (see example).

/// System properties for a Substrate-based runtime
#[derive(serde::Serialize, Deserialize, Debug, Clone, PartialEq, Eq, Default)]
#[serde(rename_all = "camelCase")]
pub struct SystemProperties {
    /// The address format
    pub ss58_format: u8,
    /// The number of digits after the decimal point in the native token
    pub token_decimals: u8,
    /// The symbol of the native token
    pub token_symbol: String,
}