Make the schema field optional

Some devices that have created fake SVDs, such as for the MSP430, may not
have schemas listed.

Co-authored-by:  =?UTF-8?q?Emil=20Gardstr=C3=B6m?=
<emil.gardstrom@gmail.com>

src/svd/device.rs

@@ -18,7 +18,7 @@ use svd::peripheral::Peripheral;
 #[derive(Clone, Debug)]
 pub struct Device {
     pub name: String,
-    schema_version: String,
+    schema_version: Option<String>,
     pub version: Option<String>,
     pub description: Option<String>,
     pub address_unit_bits: Option<u32>,
@@ -40,8 +40,7 @@ impl Parse for Device {
             name: tree.get_child_text("name")?,
             schema_version: tree.attributes
                 .get("schemaVersion")
-                .unwrap()
-                .clone(),
+                .map(|s| s.clone()),
             cpu: parse::optional::<Cpu>("cpu", tree)?,
             version: tree.get_child_text_opt("version")?,
             description: tree.get_child_text_opt("description")?,
@@ -79,7 +78,10 @@ impl Encode for Device {
         );
         elem.attributes.insert(
             String::from("schemaVersion"),
-            format!("{}", self.schema_version),
+            match self.schema_version {
+                Some(schema_version) => format!("{}", self.schema_version),
+                None => format!(""),
+            }
         );
         elem.attributes.insert(
             String::from("xs:noNamespaceSchemaLocation"),