convenient implementation TryFrom<&Option<String>> for MacAddress (#983)

## Problem

When we parse xml, the address field is optional and we use
`Option<String>` as type. For this it is convenient to have such a
TryFrom implementation as well.

## Testing

- *Added a new unit test*

rust/agama-dbus-server/src/network/model.rs

@@ -161,6 +161,57 @@ mod tests {
     use crate::network::error::NetworkStateError;
     use uuid::Uuid;
 
+    #[test]
+    fn test_macaddress() {
+        let mut val: Option<String> = None;
+        assert!(matches!(
+            MacAddress::try_from(&val).unwrap(),
+            MacAddress::Unset
+        ));
+
+        val = Some(String::from(""));
+        assert!(matches!(
+            MacAddress::try_from(&val).unwrap(),
+            MacAddress::Unset
+        ));
+
+        val = Some(String::from("preserve"));
+        assert!(matches!(
+            MacAddress::try_from(&val).unwrap(),
+            MacAddress::Preserve
+        ));
+
+        val = Some(String::from("permanent"));
+        assert!(matches!(
+            MacAddress::try_from(&val).unwrap(),
+            MacAddress::Permanent
+        ));
+
+        val = Some(String::from("random"));
+        assert!(matches!(
+            MacAddress::try_from(&val).unwrap(),
+            MacAddress::Random
+        ));
+
+        val = Some(String::from("stable"));
+        assert!(matches!(
+            MacAddress::try_from(&val).unwrap(),
+            MacAddress::Stable
+        ));
+
+        val = Some(String::from("This is not a MACAddr"));
+        assert!(matches!(
+            MacAddress::try_from(&val),
+            Err(InvalidMacAddress(_))
+        ));
+
+        val = Some(String::from("de:ad:be:ef:2b:ad"));
+        assert_eq!(
+            MacAddress::try_from(&val).unwrap().to_string(),
+            String::from("de:ad:be:ef:2b:ad").to_uppercase()
+        );
+    }
+
     #[test]
     fn test_add_connection() {
         let mut state = NetworkState::default();
@@ -457,6 +508,17 @@ impl FromStr for MacAddress {
     }
 }
 
+impl TryFrom<&Option<String>> for MacAddress {
+    type Error = InvalidMacAddress;
+
+    fn try_from(value: &Option<String>) -> Result<Self, Self::Error> {
+        match &value {
+            Some(str) => MacAddress::from_str(str),
+            None => Ok(Self::Unset),
+        }
+    }
+}
+
 impl fmt::Display for MacAddress {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let output = match &self {