fix(rsx): formatted attribute merging (#3719)

* test(rsx): add better tests for attributes merging

* fix(rsx): zero-lengthed source with non-empty segments is non-trivial

* refactor(rsx): use `is_empty` instead of `len() == 0` (clippy)

* fix(rsx): different asserts whether in release or not

packages/rsx/src/ifmt.rs

@@ -90,6 +90,11 @@ impl IfmtInput {
     }
 
     fn is_simple_expr(&self) -> bool {
+        // If there are segments but the source is empty, it's not a simple expression.
+        if !self.segments.is_empty() && self.source.span().byte_range().is_empty() {
+            return false;
+        }
+
         self.segments.iter().all(|seg| match seg {
             Segment::Literal(_) => true,
             Segment::Formatted(FormattedSegment { segment, .. }) => {