fix(cli): prevent crash when parsing stdin

When we are parsing stdin via a pipe or heredoc, the source count is 0
(unsigned) so the XML output crashes while trying to subtract from it.

crates/cli/src/parse.rs

@@ -230,13 +230,23 @@ impl ParseSummary {
     }
 }
 
-#[derive(Serialize, Debug, Default)]
+#[derive(Serialize, Debug)]
 pub struct ParseStats {
     pub parse_summaries: Vec<ParseSummary>,
     pub cumulative_stats: Stats,
     pub source_count: usize,
 }
 
+impl Default for ParseStats {
+    fn default() -> Self {
+        Self {
+            parse_summaries: Vec::new(),
+            cumulative_stats: Stats::default(),
+            source_count: 1,
+        }
+    }
+}
+
 #[derive(Serialize, ValueEnum, Debug, Copy, Clone, Default, Eq, PartialEq)]
 pub enum ParseDebugType {
     #[default]